[volt-nuts] Some questions to zeners (1N823-1N829)

[Andreas Jahn]

Hello all,
Hello Warren,

after having experimented a lot with 5V monolithic zener references
and still not found the ideal solution I want to try a 1N82x based solution.

For me a extended room temperature range of
25 degrees centigrade +/- 7 degrees (64-90 ?F) is of interest.
Since I plan to have battery supplied instruments a lower supply current would be of interest.

For the zeners a zero TC current is stated.
Over which temperature range the TC is nearly zero.
How large is the voltage deviation in the above mentioned range?

Does it play a role for the absolute temperature deviation if a 1N823 or a 1N829 is used?
Or is the behaviour equally when the individual zero TC current is used?
Is the only difference between the selections that the zero TC current is more near the 7.5mA value on the 1N829?

So is it more likely to get a low zero TC current of 4 mA on a 1N823 device than on the 1N829?
Or should I go for the 1N829A for the lowest absolute TC?

How large is the hysteresis on the zeners in a temperature range of 10-40 degrees celsius (50-104?F).
On monolithic unheated reference voltages with hermetic case I have up to around 2 ppm hysteresis difference
on temperature cycling. (see attached picture with 10-45 degrees celsius on X-Axis for an ADC with a 5V reference
measuring a LM399 heated reference over a 2:1 precision voltage divider. The ADC with the 5V reference is temperature cycled).

I blame the temperature hysteresis on the die attach to the lead frame which seems to be usually a silver filled epoxy compound.
I hope that the hysteresis on a discrete zener is much lower.

With best regards

Andreas
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[Ed Breya]

Don't bother with TC zeners, especially for battery operated equipment.
There are lots of nice IC references available from Analog Devices,
Maxim, Linear Technology, TI, an so on, that will run rings around TCZs
- you probably haven't seen the right one yet. The TCZs will only
provide near zero tempco at one temperature only, so would have to be
ovenized to get best performance. They also have an initial voltage
tolerance up to a few percent, so the circuit would need tweaks and
maybe amplification that add more error yet. I doubt that any TCZ will
match an LM399 even with very elaborate circuitry to support it.

I think the Maxim MAX3650 family would be hard to beat for a low power 5
V reference - my super-duper voltage standard (yet to built) will use
four of them averaged together, and operated at constant temperature.

If you state what kind of performance you're looking for, it would be
easier to make recommendations.

Ed

[Andreas Jahn]

Post by Ed Breya
- you probably haven't seen the right one yet.

Yes it seems to be so.

Post by Ed Breya
From my point of view this would be a LT1027BCH in a metal can housing.

But unfortunately due to ROHS they are no longer produced.

Post by Ed Breya
provide near zero tempco at one temperature only,

Do you have any measurement values to this?
Temperature is not the main issue since I want to do a temperature
correction.
But tempco should be below 1ppm/K (better 0.2ppm/K) in the 18-32 degree
celsius (64-90 ?F) range.

Post by Ed Breya
If you state what kind of performance you're looking for, it would be
easier to make recommendations.

I want to have high end specs with the power consumption of a battery
supplied DMM.
that is:
tempco below 1ppm/K
hysteresis in the 10-40 degree range well below 1ppm
no sensitivity to humidity (so all plastic housings will fall out)
ageing in the range of 1 ppm/year after some pre-ageing

With best regards

Andreas


----- Original Message -----
From: "Ed Breya" <eb at telight.com>
To: <volt-nuts at febo.com>
Sent: Saturday, January 26, 2013 7:54 PM
Subject: Re: [volt-nuts] Some questions to zeners (1N823-1N829)

Post by Ed Breya
Don't bother with TC zeners, especially for battery operated equipment.
There are lots of nice IC references available from Analog Devices, Maxim,
Linear Technology, TI, an so on, that will run rings around TCZs - you
probably haven't seen the right one yet. The TCZs will only provide near
zero tempco at one temperature only, so would have to be ovenized to get
best performance. They also have an initial voltage tolerance up to a few
percent, so the circuit would need tweaks and maybe amplification that add
more error yet. I doubt that any TCZ will match an LM399 even with very
elaborate circuitry to support it.
I think the Maxim MAX3650 family would be hard to beat for a low power 5 V
reference - my super-duper voltage standard (yet to built) will use four
of them averaged together, and operated at constant temperature.
If you state what kind of performance you're looking for, it would be
easier to make recommendations.
Ed
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and follow the instructions there.

[Михаил]

Hi, Andreas!

AJ> I want to have high end specs with the power consumption of a battery
AJ> supplied DMM.
AJ> that is:
AJ> tempco below 1ppm/K
AJ> hysteresis in the 10-40 degree range well below 1ppm
AJ> no sensitivity to humidity (so all plastic housings will fall out)
AJ> ageing in the range of 1 ppm/year after some pre-ageing

I did a lot of references with LTZ1000, LM399 and aged zeners, but
it is not easy to achieve these specs even with LTZ1000. One of the best voltage
standard Datron 4910AV (4x LTZ1000) have only 1 ppm/year drift.

Regards,
Mickle T.

[Andreas Jahn]

Hello Ed,

Post by Ed Breya
- you probably haven't seen the right one yet.

Yes it seems to be so.


----- Original Message -----
From: "Ed Breya" <eb at telight.com>
To: <volt-nuts at febo.com>
Sent: Saturday, January 26, 2013 7:54 PM
Subject: Re: [volt-nuts] Some questions to zeners (1N823-1N829)

Post by Ed Breya
Don't bother with TC zeners, especially for battery operated equipment.
There are lots of nice IC references available from Analog Devices, Maxim,
Linear Technology, TI, an so on, that will run rings around TCZs - you
probably haven't seen the right one yet. The TCZs will only provide near
zero tempco at one temperature only, so would have to be ovenized to get
best performance. They also have an initial voltage tolerance up to a few
percent, so the circuit would need tweaks and maybe amplification that add
more error yet. I doubt that any TCZ will match an LM399 even with very
elaborate circuitry to support it.
I think the Maxim MAX3650 family would be hard to beat for a low power 5 V
reference - my super-duper voltage standard (yet to built) will use four
of them averaged together, and operated at constant temperature.
If you state what kind of performance you're looking for, it would be
easier to make recommendations.
Ed
_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

[WarrenS]

Andreas

If you want to make something in volume which is just pretty good, I would
not recommend this method for a new design.
On the other hand, if this is a nuts thing to make the very best, this is
one of only a very few possible solutions.

Here are some general answers, most of my experience with these parts is
pretty dated, (i.e long ago).
As when pushing the performance limit of any reference, there is a lot of
variation between parts and even more so between manufactures.
Solution is, select, Age, select, test, grade, select.

As far as my experience with 1N823, performance depends on the run and what
is left after the manufacture has selected out the others.
With 1N823's, Yield can often go to zero. With 1N825's a typical yield I've
seen is around 25-50% (from the right manufacture and batch)

Yes the main difference is the zero TC current, with some parts there is no
zero TC current.
So yes you are more likely to get a lower current TC such as 5 ma from a
1N823 or 1N825 than a 1N829, but it could of course be > than 7.5 ma.
I don't use anything that does not have a zero TC between ~ 4 and 10 ma
I found TC to be very much a Batch thing, with up to 50% of the majority of
a batch, tending to be similar.

From a given batch, any that are considerable different, I don't use because

they may have something else wrong going on.

Another thing that needs to be selected for in high end references, and will
vary by manufacturer, is 1/F noise. The random jumps in the voltage.

For me, hysteresis has not been a issue over room temperature changes for
the most part, but something that has to be checked.
Some Manufactures are better than others, and hysteresis can and will be
effected by assembly, layout, or anything that puts any stress on the part.
Don't just solder the parts down on a PCB without a stress relieve loop in
the leads.

The zero TC current can be set so that the voltage at most any two
temperatures will be the same. (<< 1PPM)
If the voltage change in-between those two temperatures is too much, lots of
ways to add an additional second order temperature compensation.

For the best TC performance, consider the mini-oven idea with the zener,
heater resistor, and thermistor all heat shrunk together.
With a lot of outer insulation, it could be done low power by adding an
addition 0 to 5 ma to the heater resistor.

Everything has it's trade offs.
The trade off using these zeners is time and complexity.
For these parts, 5ma is about as low as you are going to get.
For low power, there are many things Much better.
The trade off you make to get low power is "Noise" & stability.
The trade off you make for the good TC of LM399 is long term stability, PPM
noise, and the high cost of selection fall outs.

The best solution will depend on many things including the desired
performance, how many you want to make, the cost you place on selection
time,
and if you can still find the 1N825's at a reasonable price like they where
in the 70s & 80s. (& $0.10 in 2000s)

For a xfer standard, the Most important criteria is 1/f random noise. Most
everything else can be compensated out.
For that, it is hard to build anything better than using a 1N825 selected
device. Plot attached.


ws

************************************

Date: Sat, 26 Jan 2013 16:30:34 +0100
From: "Andreas Jahn" <Andreas_-_Jahn at t-online.de>
Subject: [volt-nuts] Some questions to zeners (1N823-1N829)
Hello all,
Hello Warren,
after having experimented a lot with 5V monolithic zener references
and still not found the ideal solution I want to try a 1N82x based
solution.
For me a extended room temperature range of
25 degrees centigrade +/- 7 degrees (64-90 ?F) is of interest.
Since I plan to have battery supplied instruments a lower supply current
would be of interest.
For the zeners a zero TC current is stated.
Over which temperature range the TC is nearly zero.
How large is the voltage deviation in the above mentioned range?
Does it play a role for the absolute temperature deviation if a 1N823 or a
1N829 is used?
Or is the behaviour equally when the individual zero TC current is used?
Is the only difference between the selections that the zero TC current is
more near the 7.5mA value on the 1N829?
So is it more likely to get a low zero TC current of 4 mA on a 1N823
device than on the 1N829?
Or should I go for the 1N829A for the lowest absolute TC?
How large is the hysteresis on the zeners in a temperature range of 10-40
degrees celsius (50-104?F).
On monolithic unheated reference voltages with hermetic case I have up to
around 2 ppm hysteresis difference
on temperature cycling. (see attached picture with 10-45 degrees celsius
on X-Axis for an ADC with a 5V reference
measuring a LM399 heated reference over a 2:1 precision voltage divider.
The ADC with the 5V reference is temperature cycled).
I blame the temperature hysteresis on the die attach to the lead frame
which seems to be usually a silver filled epoxy compound.
I hope that the hysteresis on a discrete zener is much lower.
With best regards
Andreas

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[Andreas Jahn]

Hello Warren,

many thanks for your valuable response.

Post by WarrenS
If you want to make something in volume

The planned number of devices is more in the range of 2 up to four.

Post by WarrenS
one of only a very few possible solutions.

I guess that the others are a LM399 or a LTZ1000 based solution (both with
heaters).

Post by WarrenS
Don't just solder the parts down on a PCB without a stress relieve loop in
the leads.

Thats a good hint otherwise I will get the humidity changes of the PCB as
stress on the device.

Post by WarrenS
The zero TC current can be set so that the voltage at most any two
temperatures will be the same. (<< 1PPM)

Do you have typical values over a 64-90 ?F range. Will it be above 1ppm/K or
below?

Post by WarrenS
From your plot it would be 0.33ppm per 3 degrees in narrow range i.e. 0.1

ppm per degree.
By the way: is it degrees Fahrenheit or degrees Celsius (= 3 Kelvin)?

Post by WarrenS
From a given batch, any that are considerable different, I don't use because
they may have something else wrong going on.

Thats interesting since I have 5 pieces of the brand new LT1236AILS-5
devices.
4 of them have a tempco of 2-3 ppm/K around room temperature.
1 piece has a very flat tempco of around 0.2ppm/K (picture attached)
If I understand you right then you would not use this device because it does
behave other than the others?
On the other side it seems to be the device with the largest ageing rate of
the 5 pieces.

One experience that I did is that devices out of one batch which have a low
tempco
around room temperature tend to have a larger hysteresis and vice versa.

So I still hope that anyone has experiences with hysteresis of the zeners.

With best regards

Andreas



----- Original Message -----
From: "WarrenS" <warrensjmail-one at yahoo.com>
To: <volt-nuts at febo.com>
Sent: Saturday, January 26, 2013 9:48 PM
Subject: Re: [volt-nuts] Some questions to zeners (1N823-1N829)

Post by WarrenS
Andreas
If you want to make something in volume which is just pretty good, I would
not recommend this method for a new design.
On the other hand, if this is a nuts thing to make the very best, this is
one of only a very few possible solutions.
Here are some general answers, most of my experience with these parts is
pretty dated, (i.e long ago).
As when pushing the performance limit of any reference, there is a lot of
variation between parts and even more so between manufactures.
Solution is, select, Age, select, test, grade, select.
As far as my experience with 1N823, performance depends on the run and what
is left after the manufacture has selected out the others.
With 1N823's, Yield can often go to zero. With 1N825's a typical yield I've
seen is around 25-50% (from the right manufacture and batch)
Yes the main difference is the zero TC current, with some parts there is no
zero TC current.
So yes you are more likely to get a lower current TC such as 5 ma from a
1N823 or 1N825 than a 1N829, but it could of course be > than 7.5 ma.
I don't use anything that does not have a zero TC between ~ 4 and 10 ma
I found TC to be very much a Batch thing, with up to 50% of the majority of
a batch, tending to be similar.
From a given batch, any that are considerable different, I don't use because
they may have something else wrong going on.
Another thing that needs to be selected for in high end references, and will
vary by manufacturer, is 1/F noise. The random jumps in the voltage.
For me, hysteresis has not been a issue over room temperature changes for
the most part, but something that has to be checked.
Some Manufactures are better than others, and hysteresis can and will be
effected by assembly, layout, or anything that puts any stress on the part.
Don't just solder the parts down on a PCB without a stress relieve loop in
the leads.
The zero TC current can be set so that the voltage at most any two
temperatures will be the same. (<< 1PPM)
If the voltage change in-between those two temperatures is too much, lots of
ways to add an additional second order temperature compensation.
For the best TC performance, consider the mini-oven idea with the zener,
heater resistor, and thermistor all heat shrunk together.
With a lot of outer insulation, it could be done low power by adding an
addition 0 to 5 ma to the heater resistor.
Everything has it's trade offs.
The trade off using these zeners is time and complexity.
For these parts, 5ma is about as low as you are going to get.
For low power, there are many things Much better.
The trade off you make to get low power is "Noise" & stability.
The trade off you make for the good TC of LM399 is long term stability, PPM
noise, and the high cost of selection fall outs.
The best solution will depend on many things including the desired
performance, how many you want to make, the cost you place on selection
time,
and if you can still find the 1N825's at a reasonable price like they where
in the 70s & 80s. (& $0.10 in 2000s)
For a xfer standard, the Most important criteria is 1/f random noise. Most
everything else can be compensated out.
For that, it is hard to build anything better than using a 1N825 selected
device. Plot attached.
ws
************************************

Date: Sat, 26 Jan 2013 16:30:34 +0100
From: "Andreas Jahn" <Andreas_-_Jahn at t-online.de>
Subject: [volt-nuts] Some questions to zeners (1N823-1N829)
Hello all,
Hello Warren,
after having experimented a lot with 5V monolithic zener references
and still not found the ideal solution I want to try a 1N82x based
solution.
For me a extended room temperature range of
25 degrees centigrade +/- 7 degrees (64-90 ?F) is of interest.
Since I plan to have battery supplied instruments a lower supply current
would be of interest.
For the zeners a zero TC current is stated.
Over which temperature range the TC is nearly zero.
How large is the voltage deviation in the above mentioned range?
Does it play a role for the absolute temperature deviation if a 1N823 or a
1N829 is used?
Or is the behaviour equally when the individual zero TC current is used?
Is the only difference between the selections that the zero TC current is
more near the 7.5mA value on the 1N829?
So is it more likely to get a low zero TC current of 4 mA on a 1N823
device than on the 1N829?
Or should I go for the 1N829A for the lowest absolute TC?
How large is the hysteresis on the zeners in a temperature range of 10-40
degrees celsius (50-104?F).
On monolithic unheated reference voltages with hermetic case I have up to
around 2 ppm hysteresis difference
on temperature cycling. (see attached picture with 10-45 degrees celsius
on X-Axis for an ADC with a 5V reference
measuring a LM399 heated reference over a 2:1 precision voltage divider.
The ADC with the 5V reference is temperature cycled).
I blame the temperature hysteresis on the die attach to the lead frame
which seems to be usually a silver filled epoxy compound.
I hope that the hysteresis on a discrete zener is much lower.
With best regards
Andreas

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--------------------------------------------------------------------------------

Post by WarrenS
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[Ed Breya]

Andreas,

I think your expectations are not realistic - even if you could make
such a reference, you could not transport its voltage to the ADC without
thermoelectric effects causing error that would swamp the performance.
To keep everything below the 1 ppm/deg C range you would have to put the
entire circuit in controlled temperature - the reference, the ADC, and
the signal connection to the outside world. Constant temperature
operation would help with the overall tempco and hysteresis, but the
long term drift and noise will be intrinsic to the devices, and
unpredictable except in a statistical sense.

Ed

[John Beale]

I think your expectations are not realistic - even if you could make such a
reference, you could not transport its voltage to the ADC without
thermoelectric effects causing error that would swamp the performance. To
keep everything below the 1 ppm/deg C range you would have to put the
entire circuit in controlled temperature - the reference, the ADC, and the
signal connection to the outside world.

I don't have the practical experience or measurements to back this up, but
I understand Seebeck thermoelectric effects are a function of the
temperature difference between dissimilar-metal junctions, and not absolute
temperature. So if you have perfectly balanced both the thermal mass and
the thermal conductivity to ambient of every bimetallic junction in your
circuit, there should be zero tempco of the system due to thermocouples,
regardless of both absolute ambient temperature and the rate of temperature
change with time.

So in theory, if you use a symmetrical circuit layout with balanced thermal
mass* and then surround your battery-operated device with a large enough
block of metal (to minimize both thermal gradient, and rate of change with
time), you can get d(temperature)/d(time) of the circuit and the associated
internal temperature differentials to be arbitrarily small. How practical
this "large metal block" would be to meet a <1 ppm/C tempco spec, I do not
know. Assuming you have avoided the copper oxide problem (Cu-CuO: 1000
uV/C) the worst thermocouple will be Cu-Kovar at 40 uV/C so layout at and
around the IC packages will be the most critical.

I assume the hardest connections to keep thermally equalized would be the
terminals connecting your reference/ADC to an external device. If your
voltmeter is limited to low voltages, optimizing this suggests the smallest
and most closely-spaced connections possible, embedded in an insulating but
thermally conductive matrix (ceramic?). Standard banana jacks with 3/4 inch
spacing and surrounded by plastic, seem far from "small" or "closely
spaced" or "well thermally coupled".

* The "20-bit DAC" app note mentions this technique:
http://cds.linear.com/docs/Application%20Note/an86f.pdf

John Beale
www.bealecorner.com

[Poul-Henning Kamp]

Content-Type: text/plain; charset=ISO-8859-1
--------

Post by John Beale
So if you have perfectly balanced both the thermal mass and
the thermal conductivity to ambient of every bimetallic junction in your
circuit, there should be zero tempco of the system due to thermocouples,
regardless of both absolute ambient temperature and the rate of temperature
change with time.

This used to be very unrealistic for a host of reasons, all ultimately
terminating at the power dissipation of the various components heating
them to different degrees.

However, with todays low-power ADCs, things are starting to look
far more interesting, and given the size of the parts, submerging
the entire thing in a suitable liquid is also no longer out of the
question.

But it's still a major challenge...

But isn't that why we're on this mailing list to begin with ?

[Tony Holt]

Post by John Beale

I think your expectations are not realistic - even if you could make such a
reference, you could not transport its voltage to the ADC without
thermoelectric effects causing error that would swamp the
performance. To
keep everything below the 1 ppm/deg C range you would have to put the
entire circuit in controlled temperature - the reference, the ADC, and the
signal connection to the outside world.

Presumably, if the voltage reference uses an amplifier then a four wire
connection can be used to eliminate all the EMFs between the reference
and ADC other than those at the Kelvin connections at the ADC and
reference/amp so that the reference and the ADC don't have to be in
thermal equilibrium with each other. Could the sense wires be welded to
the ADC pins between the solder connection to the PCB and the package to
avoid the thermal EMFs of a solder joint?

Post by John Beale
I assume the hardest connections to keep thermally equalized would be
the terminals connecting your reference/ADC to an external device. If
your voltmeter is limited to low voltages, optimizing this suggests
the smallest and most closely-spaced connections possible, embedded in
an insulating but thermally conductive matrix (ceramic?). Standard
banana jacks with 3/4 inch spacing and surrounded by plastic, seem far
from "small" or "closely spaced" or "well thermally coupled"

For a couple of data points, here's one manufacturer's approach to
dealing with thermal EMFs:

http://www.hpd-online.com/reversing_switch.php

Their low thermal reversing switch uses plenty of copper and aluminium
to minimise thermal differences, claiming typical thermal offset of only
3nV, 10nV max! Its not clear (to me) though exactly where that 3nV is
being measured and how effective the 1.5mm thick copper lugs connecting
the reference source/DUT's terminals are at minimising temperature
differences between the terminals in the presence of normal air currents
in a typical Lab.

And this scanner: http://www.dataproof.com/scanner.pdf claims thermal
EMFs of less than 15nV typical (30nV max) using lots of aluminium to
keep relay contacts in thermal equilibrium.


Tony H

[Mike S]

Post by Tony Holt
Could the sense wires be welded to
the ADC pins between the solder connection to the PCB and the package to
avoid the thermal EMFs of a solder joint?

I don't think welding would make the difference, unless the wire is made
of the same material as the pin.

I'm also not clear to me how low-EMF solder helps in most cases. Solder
joints tend to be small and local, and it's the temperature difference
between the terminals which brings the thermoelectric effect into play.

For example, two copper wires soldered together - you have a Cu-solder
joint, followed by a solder-Cu joint, in very close proximity. As long
as "close" is close, and/or there's good thermal mass/conductivity,
don't the thermocouples simply offset each other?

More realistically, take a device with common tinned brass terminals on
a PC board. You have brass/tin, tin/solder then solder/copper
thermocouples in very close proximity, essentially resulting in a
brass/copper thermocouple. It seems that the temperature difference
between that connection and the similar thermocouples at the far end
device connection would overwhelm the local effects due to solder, which
require a temperature gradient across some small fraction of a mm.

Even with much larger, hand soldered terminals, something similar would
seem to apply. Wouldn't thermally insulating the terminals (which by
nature have pretty good thermal conductivity) to ensure a consistent
temperature across them be as good or better than just low EMF solder?

---
Mike

[David C. Partridge]

But if you have five solder joints on conenction to the +ve terminal and 7 on the connection to the -ve terminal at different locations on the PCB, then all bets are off ...


Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of Mike S
Sent: 28 January 2013 17:11
To: volt-nuts at febo.com
Subject: Re: [volt-nuts] Some questions to zeners (thermoelectric effects)

Post by Tony Holt
Could the sense wires be welded to
the ADC pins between the solder connection to the PCB and the package to > avoid the thermal EMFs of a solder joint?

I don't think welding would make the difference, unless the wire is made of the same material as the pin.

I'm also not clear to me how low-EMF solder helps in most cases. Solder joints tend to be small and local, and it's the temperature difference between the terminals which brings the thermoelectric effect into play.

For example, two copper wires soldered together - you have a Cu-solder joint, followed by a solder-Cu joint, in very close proximity. As long as "close" is close, and/or there's good thermal mass/conductivity, don't the thermocouples simply offset each other?

More realistically, take a device with common tinned brass terminals on a PC board. You have brass/tin, tin/solder then solder/copper thermocouples in very close proximity, essentially resulting in a brass/copper thermocouple. It seems that the temperature difference between that connection and the similar thermocouples at the far end device connection would overwhelm the local effects due to solder, which require a temperature gradient across some small fraction of a mm.

Even with much larger, hand soldered terminals, something similar would seem to apply. Wouldn't thermally insulating the terminals (which by nature have pretty good thermal conductivity) to ensure a consistent temperature across them be as good or better than just low EMF solder?

---
Mike
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[WarrenS]

This post might be inappropriate. Click to display it.

[Andreas Jahn]

Hello Warren and Volt Nuts:

for the first: from the experiences of Warren my project seems to be
feasible.
Im working now since 2008 on my "project".
Eliminating step by step the drawbacks of stability.
And learning a lot of real and not ideal parts.

I changed from SMD references with large hysteresis and bad stability to
plastic DIP devices being about a factor 3 better than SMD.
Finally finding out that some buried zener devices have relative low
hysteresis.
But air humidity was a large issue preventing stability below 10ppm over a
year.
So finally I try to use devices in hermetically tight cases which
eliminates humidity if carefully decoupled from PCB stress.

So what is feasible with selected parts and a 3rd order temperature
compensation is shown in the attached picture. (ADC13_TC)
A AD586LJ device as reference within ADC#13 is measuring a heated LM399
reference via a 2:1 capacitive divider.
X-Axis is temperature normalized to 25 degrees celsius -10/+15 degrees
measured directly at the AD586 reference.
Y-Axis on the left is LM399 measurement value in mV (divided by 2).
The red line is the measurement value of the ADC. The blue line the
resulting correction with a 3rd order polynominal.
Y-Axis on the right side is together with the green line the resulting
deviation in uV after calculating out the tempco of the reference.
Deviation is about +/-2 uV referenced to 3.4V which gives below +/-1ppm
resulting tempco after correction.
Unfortunately the slope of the uncompensated TC is relative large (about
1.1ppm). Together with my temperature resolution of about
0.1K/step this will give a relative large temperature step noise of up to
0.6uV/step in the 5V-Range of the ADC.
But anyway ADC #13 was the first ADC which I can use for ageing
measurements.

After a run in phase of nearly 1 year the ageing of ADC #13 stabilized.
Currently I compare ADC13 nearly every day with 3 heated references (1 LM399
= LM_2 and 2 LTZ1000A = LTZ_1/2).
The last half year the ageing is about 0.5 to 1.5 ppm for 6 months compared
to the heated references.
See picture ADC13_longterm:
X-Axis is day
Y-Axis left is drift in ppm with red = LM399#2, green = LTZ1000A #1, blue =
LTZ1000A #2
Y-axis right is temperature in degree celsius of the temperature sensor near
ADC13 reference.

By the way: up to now I could not measure any effect which is related to
thermocouples.
Ok my temperature step noise is still too high. And probably I am using the
wrong connectors in my tests:
cheap D-Sub connectors where a metal shield is equalizing the temperature of
2 relative close neighboured contacts.

With best regards

Andreas


----- Original Message -----
From: "WarrenS" <warrensjmail-one at yahoo.com>
To: <volt-nuts at febo.com>
Sent: Sunday, January 27, 2013 11:23 PM
Subject: [volt-nuts] Some questions to zeners (1N823-1N829)

Post by Andreas Jahn
Andreas
1st my two cents worth concerning comments of others that are being more
practical than nutty.
"it is not easy to achieve these specs",
Too True,
If you do not already know this, then I'd suggest you back off and do some
major readjustment of your requirements.
Assuming you already know it is not easy and why, then
IMHO, what you want to make is doable with the right compromises, some of
which being
A little extra power.
a lot of extra time
a lot of test selecting
some extra circuits or S/W
"your expectations are not realistic "
True, if you think it can be done from a data sheet and a few of off the
shelf parts in a production device.
Not true, if you want to go totally nuts and have the time to do it.
"The thermoelectric effects causing error would swamp the performance"
Could be true for some. If you do not already know enough to keep these
effects well under <<0.1PPM then should find another starter project.
"To keep everything below the 1 ppm/deg C range you would have to put the
entire circuit in controlled temperature"
Too true, But for these requirements, fortunately you do not have to keep
*Everything* under 1 ppm/deg C
ALL you have to do is to keep the sum total of Everything under 1ppm. Big
difference!
And that is very easy to do, by a fact of ten plus if desired.
"long term drift and noise will be intrinsic to the devices, and
unpredictable except in a statistical sense."
Half true,
Using the *right device*, and a lot of time, noise can be low enough to be
mostly insignificant
and long term drift will follow a predicable slope.
It ain't easy and it's going to take a lot of time. If you think
otherwise, you need a new project..
"provide near zero tempco at one temperature only"
Answer too limited to be useful
zero TCing a circuit, can provide no voltage difference at any two
temperatures, (to the limit of the repeatability)
Depending how flat you need it between these two temperatures, is the only
issue.
Best to plan on having a second order TC method as well.
A total max total deviation of 1ppm and a 0.1 PPM /deg is not too hard to
get.
"One of the best voltage standard Datron 4910AV (4x LTZ1000) have only 1
ppm/year drift"
OK, so that shows there is at least one way to do it, so it is not
impossible.
Now just need to find the best way for you to do it.
"Don't bother with TC zeners"
Not a bad idea, but Unfortunately Not a lot of other choices considering
your requirements.
The other choice you have (that you should consider if time is a big issue)
is to get some three terminal Fluke voltage references.
If necessary by removing them from old test equipment such as Fluke 731
Those have already been selected and aged, and you'd just then need to
work on the long term drift compensation selection method.
"There are lots of nice IC references available"
True, and if you can live with data sheet specs, they are much better
choices.
But hard if not impossible to find anything that will compare to 1/F pop
corn noise, and long term stability
"I doubt that any TCZ will match an LM399"
True when considering a wider temperature range, and it sure makes things
easier,
so a good suggestion if TC was your main problem.
But TC need not be a problem, and most any good zener will outperform most
any selected LM399 in low freq noise and stability.
"so would have to be ovenized to get *best* performance".
True of Any circuit when "best" TC is concerned, so not very relevant,
The question is can it be made good enough without an oven?
And the answer is defiantly yes with the right tempco circuit.
And if you want it even better, can make a very low power mini-oven.
Back to your email ******************

Post by Andreas Jahn
Do you have typical values over a 64-90 ?F range. Will it be above 1ppm/K
or below?

I do not have any data over that Temp range in front of me.
Over My room changes (about 1/2 that temp range), can keep the Total
voltage error down to around 1PPM, (not just /deg)
If it is an important consideration, I may try and see what happens over a
wider range.
For a backup plan, considering having an addition second order TC
compensating method such as S/W.
Overall compensating to 1ppm / C is no problem,
which is Total change +- 7PPM over a +- 7 deg C range
Ten times better is possible over narrow temp ranges like that.
It is the hysteresis and stability that is going to be a limiting factor,
and last I looked,
The zeners I tested had no measurable Hysteresis over a much wider temp
range than that.

Post by Andreas Jahn

Post by WarrenS
From your plot it would be 0.33ppm per 3 degrees in narrow range i.e. 0.1
ppm per degree.

Correct, but not a relevant measurement.
ANY straight line TC as seen there can be zeroed out.
That was just a pre-test plot to see if the part was low enough noise to
do a more accurate Zero TC.

Post by Andreas Jahn
By the way: is it degrees Fahrenheit or degrees Celsius (= 3 Kelvin)?

My world is mostly in F, and since I was not even plotting temp at the
same time, pretty irrelevant for that data set.

Post by Andreas Jahn
If I understand you right then you would not use this device because it
does
behave other than the others?

Your understaning is not correct, not even close. My comment applies to a
preselection process of 1N82x parts.
In the case you stated, your preselection process (assuming you have
enough to pick from) would be to select all the parts that are less than
say 1PPM, with maybe a 20% yield.
If the yield is too low then make a second pass for parts that are say 2
+-1 PPM,
and in that case, then yes the "best" part would not go into that batch,
because it will likely need a different compensation than the others.

Post by Andreas Jahn
On the other side it seems to be the device with the largest ageing rate
of the 5 pieces.

likely too little data to be important, Just as likely random luck as
anything important.
Until after you do some pre-aging, (whatever that may mean for that part)
I would not even bother looking at that data this early on in the
selection process.

Post by Andreas Jahn
So I still hope that anyone has experiences with hysteresis of the zeners.

Turn on after power down, and hysteresis, repeatable, etc altogether is
under 1PPM.
As part of my pre-test TC procedure, I hit them with cold spray to 0 C and
heat them with a heat gun to ~50C, a few times,
If they are not repeatable they are not used in further testing.
I have a question about two of your requirements
AJ> tempco below 1ppm/K
AJ> hysteresis in the 10-40 degree range well below 1ppm
This suggest to me that what you are really planning on making is
something with an overall compensted TC that is well below a 1 ppm/K,
otherwise your hysteresis requirement does not make a lot of sense to me.
So if what you are really after is more like < 1 PPM total error over time
and temp, all this extra trouble now makes a lot more sense.
Now the disclaimer, I have no idea if any of the now available 1N825 or
better parts work like I've described.
It is VERY much a manufacture sensitive thing.
Short term noise being the biggest rejecting thing I've seen in the past.
0.5 PPM (3uv steps) are not untypical for some batches.
If someone that has enough of these parts from a single batch and
manufacture to make it worth while testing them, and will make them
available to others, then I'll test and report the results and compare
them to what I have.
There may be another less direct way to get where you want to go (You did
not say where that was)
Divide the task into two or more sections.
As an example, One being the low power short term stable DVM device that
works over a limited temperature.
There would be no problem getting 0.1 PPM accuracy/repeatable for that part.
and a second higher power device that is mostly off, and powered up Just
long enough to calibrate the low power xfer DVM above.
And if you can then include some sort of cal exchange program for the
higher powered but less used device, this would make a very hard project
into a relative easy project.
ws
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[Михаил]

Hi, Andreas!

AJ> But anyway ADC #13 was the first ADC which I can use for ageing
AJ> measurements.

Where to find information about this ADC? It is very interesting thing. Is it
Multi-slope III, PWM, or another type?

Regards,
Mickle T.

[Poul-Henning Kamp]

Content-Type: text/plain; charset=ISO-8859-1
--------
In message <AA473A4936944A1199FC6F4C9C5D9A1A at laptop>, "Andreas Jahn" writes:

Amazing!

Recently I came across a very interesting ADC from TI, designed for
use with geophones: ADS1282

It might be interesting for volt-nuttery: It samples 2 bits at
4MHz, which are then downsampled to 31 bits at 4kHz or less. For
this reason it is incredibly linear and has very good S/N.

[Тимофеев Михаил]

The ADS1282 offers outstanding specification, but not very useful in the wide-scale DVM application range due to low input and reference voltages.
Within a year I'm trying to develop a design of the selfcal 7.5-digits DMM with ADS1282 ADC. Reference (+/- 10V) is a aged 1N829A with 0.03ppm noise and 2nd order TC correction, downscaled to +/-2.5 V with zero TC divider.

Mickle T.

Post by Poul-Henning Kamp
Content-Type: text/plain; charset=ISO-8859-1
--------
Amazing!
Recently I came across a very interesting ADC from TI, designed for
use with geophones: ADS1282
It might be interesting for volt-nuttery: ?It samples 2 bits at
4MHz, which are then downsampled to 31 bits at 4kHz or less. ?For
this reason it is incredibly linear and has very good S/N.
--
Poul-Henning Kamp ??????| UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG ????????| TCP/IP since RFC 956
FreeBSD committer ??????| BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

[Andy Bardagjy]

The ADS1282 is a really cool part! After studying the ADS1282
datasheet is appears that *at best* it can achieve 130dB SNR which, by
my calculations is effectively 21.3 bits - by the formula
SNR=(6.02N+1.786)dB.

This got me thinking, are there any higher performing "24 bit" ADCs we
should look at for our "nuttery". One that I found is the Linear
LTC2440 (and others in the family). It claims, at its highest
resolution, 24.6 effective bits. That said, I am having trouble
comparing SNR directly. I need to read more to understand how to
convert its noise spec - 200nV RMS noise - to a SNR dB figure.

Andy Bardagjy
bardagjy.com

Post by Тимофеев Михаил
The ADS1282 offers outstanding specification, but not very useful in the wide-scale DVM application range due to low input and reference voltages.
Within a year I'm trying to develop a design of the selfcal 7.5-digits DMM with ADS1282 ADC. Reference (+/- 10V) is a aged 1N829A with 0.03ppm noise and 2nd order TC correction, downscaled to +/-2.5 V with zero TC divider.
Mickle T.

Post by Poul-Henning Kamp
Content-Type: text/plain; charset=ISO-8859-1
--------
Amazing!
Recently I came across a very interesting ADC from TI, designed for
use with geophones: ADS1282
It might be interesting for volt-nuttery: It samples 2 bits at
4MHz, which are then downsampled to 31 bits at 4kHz or less. For
this reason it is incredibly linear and has very good S/N.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

_______________________________________________
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

[beale]

-------Original Message-------
From: Poul-Henning Kamp <phk at phk.freebsd.dk>
...a very interesting ADC from TI, designed for use with geophones: ADS1282

31 bits is impressive. With the +/-2.5 V reference I gather the maximum input range in differential mode is Vref/2 or +/- 1.25 V. The Voffset drift is 0.02 uV/C "typical" and the INL is 0.5 ppm "typical", but 4 ppm "max". For comparison, the 24-bit Analog Devices AD7190 claims typical Voffset = 0.1 uV/C, or 0.005 uV/C in chop mode, and INL = 1 ppm typ. and 5 ppm max, with a maximum differential input voltage of +/-5 V. So the AD7190 has 2x worse INL (typ.), but with a 4x larger input voltage range, the INL is 2x better (typical) and almost 4x better (max) when measured in microvolts.

For quantity 1 purchase, the AD7190 is $11 (Newark/Element14) and the ADS1282 is $62 (Arrow). For $62, you can get a complete eval board for the AD7190 which works as a 4-channel input standalone USB-powered voltmeter (although the eval board's ADR421 2.5V reference is not volt-nut quality.) By the way, the 7190 chip provides for two selectable, independent differential Vref inputs, which are separate from the two differential or four pseudo-differential signal inputs.

[Tony Holt]

Post by Andreas Jahn
After a run in phase of nearly 1 year the ageing of ADC #13 stabilized.
Currently I compare ADC13 nearly every day with 3 heated references (1
LM399 = LM_2 and 2 LTZ1000A = LTZ_1/2).
The last half year the ageing is about 0.5 to 1.5 ppm for 6 months
compared to the heated references.
X-Axis is day
Y-Axis left is drift in ppm with red = LM399#2, green = LTZ1000A #1,
blue = LTZ1000A #2
Y-axis right is temperature in degree celsius of the temperature
sensor near ADC13 reference.
By the way: up to now I could not measure any effect which is related
to thermocouples.
Ok my temperature step noise is still too high. And probably I am
cheap D-Sub connectors where a metal shield is equalizing the
temperature of 2 relative close neighboured contacts.
With best regards
Andreas

Andreas,

Very interesting results - thanks for sharing your painstaking work.
Hope you don't mind me asking a few questions though:

How are you dealing with the issue of drift in the thermocouple
measurements (including the cold junction compensation)? Do you
calibrate it periodically? Thermocouples aren't noted for high stability
- but presumably at room temperature its perhaps not much of an issue.

Do you know what temperature the LTZ1000 references are operating at,
and how long have they been operated for - ie. have they been aged prior
to starting the long term test? (Presumably the answer to that is the
fact that you are showing results from day 460 onwards?)

Have you any insight into how stable the ADC's reference (AD586LJ) is?
I.E. Have you made any occasional or periodic measurements with other
calibrated instruments during the long term test or is it the long term
test results themselves which leads you to state: "After a run in phase
of nearly 1 year the ageing of ADC #13 stabilized."?

Thanks, Tony H

[Andreas Jahn]

Hello Tony,

I do not understand your question with thermocouples.
The temperature measurement within ADC13 is done by a NTC.
The thermocouple discussion came up because of the widely used banana plugs
on instruments.

Most of your questions to LTZ1000 have been answered mid to late 2010 within
this board.
Day zero of this diagram is the day when I started automatic measurement
sequences with a relay multiplexer.
ADC13 was included around day 200. Some weeks after first powering and after
doing all adjustments.
The whole story of ADC13 can be seen in the diagram.
First ageing around 3.4 ppm/khr (light blue) referenced to first powering.
Second ageing 2.4 ppm/khr (pink) due to loading the reference by 15mA over
night.
After some weeks ageing slowed down suddenly. So I terminated the 15mA
loading.

My world are the 3 references 1 LM 399 and 2 LTZ1000A.
I compare them annually to 2 Keithley 2000.
Drift is within 1 count (10uV) over 1 to 2 years. (same direction on both
Keithleys)
LM399 seems to drift upwards. The 2 LTZ1000 seem to drift slightly
downwards.

Out of 29 AD586LQ I found about 3-4 which have usable parameters.

With best regards

Andreas

----- Original Message -----
From: "Tony Holt" <vnuts at toneh.demon.co.uk>
To: <volt-nuts at febo.com>
Sent: Wednesday, January 30, 2013 10:49 AM
Subject: Re: [volt-nuts] Some questions to zeners (1N823-1N829)

Post by Ed Breya

Post by Andreas Jahn
After a run in phase of nearly 1 year the ageing of ADC #13 stabilized.
Currently I compare ADC13 nearly every day with 3 heated references (1
LM399 = LM_2 and 2 LTZ1000A = LTZ_1/2).
The last half year the ageing is about 0.5 to 1.5 ppm for 6 months
compared to the heated references.
X-Axis is day
Y-Axis left is drift in ppm with red = LM399#2, green = LTZ1000A #1, blue
= LTZ1000A #2
Y-axis right is temperature in degree celsius of the temperature sensor
near ADC13 reference.
By the way: up to now I could not measure any effect which is related to
thermocouples.
Ok my temperature step noise is still too high. And probably I am using
cheap D-Sub connectors where a metal shield is equalizing the temperature
of 2 relative close neighboured contacts.
With best regards
Andreas

Andreas,
Very interesting results - thanks for sharing your painstaking work. Hope
How are you dealing with the issue of drift in the thermocouple
measurements (including the cold junction compensation)? Do you calibrate
it periodically? Thermocouples aren't noted for high stability - but
presumably at room temperature its perhaps not much of an issue.
Do you know what temperature the LTZ1000 references are operating at, and
how long have they been operated for - ie. have they been aged prior to
starting the long term test? (Presumably the answer to that is the fact
that you are showing results from day 460 onwards?)
Have you any insight into how stable the ADC's reference (AD586LJ) is?
I.E. Have you made any occasional or periodic measurements with other
calibrated instruments during the long term test or is it the long term
test results themselves which leads you to state: "After a run in phase of
nearly 1 year the ageing of ADC #13 stabilized."?
Thanks, Tony H
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[WarrenS]

Andreas

Nice work.
May I suggest one of the next steps you work on is improving the short term
noise and resolution over say a couple hrs, when at constant temperature, by
a factor of ten or so, as ultimately that will be what limits the
performance.

For the long term ageing problem, which will be your major issue after you
get all the 'easy' things worked out.
Make that a separate issue by using a separate unpowered part(s), i.e don't
use the working ADC ref.
Power the long term ageing part up only as needed to auto cal the ADC.
With the long term reference mostly off, this can significantly help (fix)
your power and ageing issue.
That will make the near impossible task of finding a low power, low long
term drift voltage ref much easier.
And yes, it will have to be a part with good turn on repeatable and
hysteresis.
But that will be a heck of a lot easer than estimating the long term ageing
rate of a constant on part that is in an uncontrolled environment to 1PPM /
year.

For the long term reference part, one thing to consider is to use the
average of say 10 aged and selected zeners.
The power on duty cycle can be made so low (<<< 10%) that you can afford
the 75 ma or so they take when on.


ws

********************
*******************
Hello Warren and Volt Nuts:

for the first: from the experiences of Warren my project seems to be
feasible.
I'm working now since 2008 on my "project".
Eliminating step by step the drawbacks of stability.
And learning a lot of real and not ideal parts.

... snip

With best regards

Andreas

[Andreas Jahn]

Unfortunately the original message has been filtered out by the large
attachment filter.
So sorry for the bad quality of the reszized picture.

----- Original Message -----

Hello Mickle,

ADC #13 is just my "serial number" of ADCs
Up to now all (#1-#14) are based on the 24 bit sigma delta LTC2400.

On the photo the left pcb has:
lower left photocoupler to isolate from PC (RS232 either direct or via USB
serial cable).
upper left PIC12F675 for reading ADC + storage of all calibration constants
like
nominal reference voltage, 3rd order temperature correction,
2nd order linearity correction, serial number etc.
upper right: AD586LQ. Note that only Pin 4 is soldered directly to the pcb.
Other pins with Vero wire to remove PCB stress (humidity) from the
reference.
The orange part at the AGND pin is the temperature NTC.
lower right: LTC2400.

Foreground: voltage stabilisation with low noise LT1763 to 14V (minimum
needed for AD586)

right PCB:
Capacitive 2:1 voltage divider with LTC1043 + LT1050 buffer.
The noise from ADC input is isolated from LT1050 buffer
by a R/C low pass. Otherwise the switching from ADC input will
increase noise level well above the datasheet spec.

When measuring all (ADCs + other references) is mounted on a large metal
ground plane.
Clothes over the ADCs + connections keep air currents away.

Since the noise level of LTC2400 is rather high, I have to use large
integration times
(averaging several hundred measurements over 1 or 5 minutes) to reduce
the noise from a single measurement of 10uVpp (0.3ppm RMS in the datasheet)
to below 1uVpp.

For further experiments I will probably have to change to a other ADC with
less noise
like LTC2440 or AD7190.

But the LTC2400 has some advantages that I'm missing on other ADCs.
first is the +/-12.5% (or at least some millivolts when regarding linearity)
overrange without clipping of the ADC-value.
This makes offset + full scale calibration more easy than on other devices
which are clipping the reading at zero + full scale.
The next is the very low gain (0.02ppm/K) + offset drift (0.01ppm/K)
compared to other sigma delta converters.
And finally the 4 sub-bits below the 24 bits (giving a readout of totally 28
bits) would help
on a low noise ADC like AD7190 to get a more gaussian distribution of the
values.

With best regards

Andreas


----- Original Message -----
From: "Mickle" <timka2k at yandex.ru>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Tuesday, January 29, 2013 8:31 AM
Subject: Re: [volt-nuts] Some questions to zeners (1N823-1N829)

Post by Михаил
Hi, Andreas!
AJ> But anyway ADC #13 was the first ADC which I can use for ageing
AJ> measurements.
Where to find information about this ADC? It is very interesting thing. Is
it
Multi-slope III, PWM, or another type?
Regards,
Mickle T.
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[beale]

From: Andreas Jahn <Andreas_-_Jahn at t-online.de>
...the 4 sub-bits below the 24 bits (giving a readout of totally 28 bits) would help on a low noise ADC like AD7190 to get a more gaussian distribution of the values.

Thanks for the well-explained photo Andreas, I guess you etched your own PCBs?

By the way, the data sheet for the LTC2440 shows that it sends a 32 bit word, including 4 LSBs beyond the 24 bit result. I found that while those bits are logically present, only two of them are "real" data bits, the last two are fixed. b1 is always low and b0 is always high. I contacted Linear Tech support and they confirmed it:

"I ran your schematics and questions through our applications engineer and he mentioned what you are seeing is normal. The two LSBs are far below the noise floor and have no real influence but I agree this should be clarified in the datasheet." -Uma D, Product Marketing Engineer, Linear Tech Nov.5 2012

If enclosed PNG image makes it, the scope screenshot shows an example. Data is valid on the rising edge of the clock. Light blue traces on the DATA channel (2) show previous data words (scope display-persist mode).

Both the LTC2440 and AD7190 can run at higher output rates, with higher noise. So, for example if you run at 10 Hz output rate (or even 50 Hz or 60 Hz, depending on your local AC frequency) and then do your own averaging, I think you can get useful data beyond 24 bits if your Vin / Vref is clean enough.

I previously sent Andreas a screen capture of the AD7190 eval board software which included a lopsided histogram, but that was in fact a software problem on the PC host (LT was sloppy in making their LabView app). For histograms with only a few bins, the labview auto-scale function sometimes crops the right-hand edge off the histogram. You see this clearly if you watch it acquiring data live. If you export the raw AD7190 data and look at it in an external graphing program, you can confirm the histogram is symmetric as expected, for any large enough dataset.

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[Andreas Jahn]

Hello John,

Yes this PCB is etched by myself.
Good to know that the LTC2440 does behave somewhat different in transferring
the data than my LTC2400.
I have looked to the data and on the LTC2400 all 4 sub LSBs are toggling.

To the 50 or 60 Hz output rate:
The output rate is not necessarily equal to integration time.
When I measure the input of a LT2400 with a oscilloscope,
I see the swiching noise from sigma delta converter
only the 2nd half 80 ms out of 160 ms output rate (configured for 50 Hz).
The first half of the measurement time the input is passive.

I guess that the LTC2400 does a kind of self-calibration every measurement
cycle.
Otherwise he would not have the offset and gain drift specs.

Good also to know that the AD7190 has gaussian noise.
So I will have to look up the two devices for a further ADC.

With best regards

Andreas


----- Original Message -----
From: "beale" <beale at bealecorner.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Wednesday, January 30, 2013 11:46 PM
Subject: Re: [volt-nuts] bits beyond 24 from the ADC

Post by beale

From: Andreas Jahn <Andreas_-_Jahn at t-online.de>
...the 4 sub-bits below the 24 bits (giving a readout of totally 28 bits)
would help on a low noise ADC like AD7190 to get a more gaussian
distribution of the values.

Thanks for the well-explained photo Andreas, I guess you etched your own PCBs?
By the way, the data sheet for the LTC2440 shows that it sends a 32 bit
word, including 4 LSBs beyond the 24 bit result. I found that while those
bits are logically present, only two of them are "real" data bits, the
last two are fixed. b1 is always low and b0 is always high. I contacted
"I ran your schematics and questions through our applications engineer and
he mentioned what you are seeing is normal. The two LSBs are far below the
noise floor and have no real influence but I agree this should be
clarified in the datasheet." -Uma D, Product Marketing Engineer, Linear
Tech Nov.5 2012
If enclosed PNG image makes it, the scope screenshot shows an example.
Data is valid on the rising edge of the clock. Light blue traces on the
DATA channel (2) show previous data words (scope display-persist mode).
Both the LTC2440 and AD7190 can run at higher output rates, with higher
noise. So, for example if you run at 10 Hz output rate (or even 50 Hz or
60 Hz, depending on your local AC frequency) and then do your own
averaging, I think you can get useful data beyond 24 bits if your Vin /
Vref is clean enough.
I previously sent Andreas a screen capture of the AD7190 eval board
software which included a lopsided histogram, but that was in fact a
software problem on the PC host (LT was sloppy in making their LabView
app). For histograms with only a few bins, the labview auto-scale
function sometimes crops the right-hand edge off the histogram. You see
this clearly if you watch it acquiring data live. If you export the raw
AD7190 data and look at it in an external graphing program, you can
confirm the histogram is symmetric as expected, for any large enough
dataset.

--------------------------------------------------------------------------------

Post by beale
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[David]

The data sheets for the LTC2400 and LTC2440 are very specific that
offset and full-scale calibrations are performed on every conversion
cycle.

On Sat, 2 Feb 2013 14:58:19 +0100, "Andreas Jahn"

Post by Andreas Jahn
. . .
The output rate is not necessarily equal to integration time.
When I measure the input of a LT2400 with a oscilloscope,
I see the swiching noise from sigma delta converter
only the 2nd half 80 ms out of 160 ms output rate (configured for 50 Hz).
The first half of the measurement time the input is passive.
I guess that the LTC2400 does a kind of self-calibration every measurement
cycle.
Otherwise he would not have the offset and gain drift specs.
. . .