Stacking AD587s for higher output voltage? (20V, 30V, etc.)

[TimNJ]

Hi everyone,

I've been looking at some voltage reference ICs for a project I've been thinking about. In the REF102 datasheet, one of the application circuits is a stacked array (series arrangement) of REF102s to produce output voltages that are multiples of 10V. I think I prefer the AD587 for long term stability, so I was curious if there would be any reason I couldn't do that with a few AD587s.

On the same token, would the initial error, long term stability, and temperature stability go up by a factor of N, the number of references? At least as an estimate?

Thanks!

Tim

AD587: http://www.analog.com/media/en/technical-documentation/data-sheets/AD587.pdf

[ap]

Statistically, aging drift ratio and tempco go down by a factor of sqrt(N).

[Kleinstein]

Noise will go down as sqrt(N). However aging drift, pressure effect and also much of the tempco (especially the higher order contributions) tend to be very similar between the units. So there will be only a limited advantage from these. However an amplifier to make 20 / 30 V from 10 V can easily have large extra errors, and these can be avoided.

[lars]

Don´t forget the humidity sensitivity of epoxy packages. These will also not cancel each other.

Lars

[TimNJ]

Noise will go down as sqrt(N). However aging drift, pressure effect and also much of the tempco (especially the higher order contributions) tend to be very similar between the units. So there will be only a limited advantage from these. However an amplifier to make 20 / 30 V from 10 V can easily have large extra errors, and these can be avoided.

Thanks a lot. So, because noise is intrinsically random, it would be reduced (statistically)? But the distribution of the other factors might be narrower (and with an offset) so the combined tempco/age drift would likely be similar to just one reference?

Do you have any insight into how a zero-drift op-amp, like an LTC2057, with an amplification of 2x, would compare to two stacked AD587 refs?

Don´t forget the humidity sensitivity of epoxy packages. These will also not cancel each other.

Lars

I think I would use the ceramic package but thank you for that insight.

[Kleinstein]

For an x2 amplifier, usually the OP is not the critical part. The difficult parts are often the resistors - 2 well matched resistors can be more expensive than the OP. An LTC2057 would not cause a significant offset drift - however it might cause trouble from higher frequency spikes. So it might not be the best choice.

Noise of the different refs is uncorrelated and thus ads as power and thus the square root of N law for many refs. The noise some times adds and sometimes subtracts. So the sum of two uncorrelated noise sources is not twice the voltage, but only twice the power and thus sqrt(2) times the voltage.

The other factors like temperature, aging, pressure are likely highly correlated. So here the relative drift will be likely be only a little better, if at all. The TC and so on will be the average value of the separate refs and it is highly likely the TCs and similar coefficients will have the same sign on all the refs. So chances are the errors can not compensate.

[TimNJ]

For an x2 amplifier, usually the OP is not the critical part. The difficult parts are often the resistors - 2 well matched resistors can be more expensive than the OP. An LTC2057 would not cause a significant offset drift - however it might cause trouble from higher frequency spikes. So it might not be the best choice.

Noise of the different refs is uncorrelated and thus ads as power and thus the square root of N law for many refs. The noise some times adds and sometimes subtracts. So the sum of two uncorrelated noise sources is not twice the voltage, but only twice the power and thus sqrt(2) times the voltage.

The other factors like temperature, aging, pressure are likely highly correlated. So here the relative drift will be likely be only a little better, if at all. The TC and so on will be the average value of the separate refs and it is highly likely the TCs and similar coefficients will have the same sign on all the refs. So chances are the errors can not compensate.

Thanks for your detailed explanation.

Ah yes, I suppose the switching noise of the zero-drift amp could be problematic for a high precision/metrology project. Perhaps I could just use a filter? Reduce the bandwidth of the amp but I'm really only trying to amplify DC.

I suppose I could use two very low tempco-resistors (but not necessarily high precision), and then use a small series pot to micro-adjust the output voltage with a calibrated 7-1/2 or 8-1/2 digit meter?

This might be something to investigate empirically on the bench. That is, comparing a chopper/zero-drift amp against a normal op-amp over time/temperature.

[Andreas]

Hello,

this should also work with AD587.
But consider that the AD587 needs more headroom. (at least 3.5V)
So with 3 stacked references you will need at minimum 33.5V.

Further you have references which sink current in the stack. So the load regulation will be worse at least for those references.

Noise will go down as sqrt(N).

Think twice: this is only valid for paralleling the outputs.
In series connection the noise adds with sqrt(N).
So only the relative noise (noise per volt output voltage) will be reduced.

with best regards

Andreas

[TimNJ]

Hello,

this should also work with AD587.
But consider that the AD587 needs more headroom. (at least 3.5V)
So with 3 stacked references you will need at minimum 33.5V.

Further you have references which sink current in the stack. So the load regulation will be worse at least for those references.

Noise will go down as sqrt(N).

Think twice: this is only valid for paralleling the outputs.
In series connection the noise adds with sqrt(N).
So only the relative noise (noise per volt output voltage) will be reduced.

with best regards

Andreas

Thank you!

Is there any reason that an AD587 (or other) feeding a zero-drift op-amp with a gain of 3 (or any value) would not work just as well? Perhaps I'm reading the spec sheets wrong, but noise doesn't seem like it would be a problem. Additionally, an offset incurred by nature of the op-amp should be able to be nulled out with a offset adjustment pot...I think(?) That's all assuming I have a known good meter to calibrate this against.

On that note, I do not have a suitable meter at the current time but am considering a Keithley 2010 with 7-1/2 digits/ 0.0025% (1yr) basic DCV accuracy. It's about the best I can afford. If I calibrate my metrology circuit using a cal'd DMM spec'd at 0.0025%, then is my circuit effectively 0.0025% accurate? Do I also add my worst case ppm drift over time and temperature to that? Let's assume that's (somewhat correct) and the absolute accuracy of my circuit is 0.0035%. If I want to calibrate another meter using my reference circuit, what level of accuracy should the target meter be in comparison to my reference. 1/2, 1/5, 1/10? I've seen Fluke cite 1/5 but not sure where that came from. This is something that has confused me for a while and can't wrap my head around.

Anyway, if anyone could shed some light on those ideas or point me to some literature that would help me understand, I'd really appreciate it. Thank you again for all your help.

[ap]

First of: Yes, sure, square law decrease is all (noise, drift) relative to the output voltage. Otherwise one could build a zero noise voltage ultra high resistor by series connection of many low ohm resistors... Semicondutor drift (special effects such as general drift direction caused e.g. by case stress effects put aside) can be considered as very low frequency noise (noise measurements are therefore sometimes used to predict drift rates; there are some dosc available about this).
Re. combining measurement uncertainties, the GUM is a good guideline:
www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf

[Kleinstein]

Using an amplifier adds errors of the amplifier to the errors of the reference. So the relative error will always be larger than that of the reference. With 2 or 3 references the relative error will be about the same as a single reference, maybe even slightly better.  Noise is better with 3 refs in series than amplifying a reference. It depends on the application if noise is really relevant.

For the amplifier, the modern OPs are usually not a large source of error anymore, but the resistors are still not perfect. These will add to the drift.

Especially with a 30 V reference, it is difficult to measure this directly with a DMM: the usually meters are not very good at 30 V, because they usually start with a divider in this range. Many of the better meters can measure 10 V much better, thus one could measure the 3 source at 10 V and add the numbers to get a considerably better result than a direct 30 V reading.  With most precision meters the specs in the 10 V (or maybe 20 V) range are much better than for the higher (e.g. 100 V) ranges.

[d-smes]

I tried this with AD581's.  I added a load resistor to the bottom stage and set load current slightly higher than the quiescent current such that all stages source current (AD581 can only sink 5 uA).  With two stages it was OK but when I went to many stages, it oscillates.  I then set it up so each reference had R-C decoupling on its +Vs pin.  Still oscillates.  I set it aside for another day.   

[TimNJ]

First of: Yes, sure, square law decrease is all (noise, drift) relative to the output voltage. Otherwise one could build a zero noise voltage ultra high resistor by series connection of many low ohm resistors... Semicondutor drift (special effects such as general drift direction caused e.g. by case stress effects put aside) can be considered as very low frequency noise (noise measurements are therefore sometimes used to predict drift rates; there are some dosc available about this).
Re. combining measurement uncertainties, the GUM is a good guideline:
www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf

Thank you for that valuable link. My thoughts are quite scattered about characterizing a new piece of metrology equipment based off of the stated accuracy of its calibrator.

Using an amplifier adds errors of the amplifier to the errors of the reference. So the relative error will always be larger than that of the reference. With 2 or 3 references the relative error will be about the same as a single reference, maybe even slightly better.  Noise is better with 3 refs in series than amplifying a reference. It depends on the application if noise is really relevant.

For the amplifier, the modern OPs are usually not a large source of error anymore, but the resistors are still not perfect. These will add to the drift.

Especially with a 30 V reference, it is difficult to measure this directly with a DMM: the usually meters are not very good at 30 V, because they usually start with a divider in this range. Many of the better meters can measure 10 V much better, thus one could measure the 3 source at 10 V and add the numbers to get a considerably better result than a direct 30 V reading.  With most precision meters the specs in the 10 V (or maybe 20 V) range are much better than for the higher (e.g. 100 V) ranges.

I suppose I have to come up with an allowable noise figure. I'm trying to calibrate 4-1/2 digit meters (and maybe 5-1/2 meters depending on how this all goes). I'm not exactly sure what is realistic/attainable. Maybe 10-25uVp-p? I have much more reading to do, to figure out what's necessary and how to accurately measure such low noise levels.

The reason I want to be able to get up to ~30V is a handful of older (but still popular meters) want to be calibrated at 19V, 29V, 30V, etc.

I tried this with AD581's.  I added a load resistor to the bottom stage and set load current slightly higher than the quiescent current such that all stages source current (AD581 can only sink 5 uA).  With two stages it was OK but when I went to many stages, it oscillates.  I then set it up so each reference had R-C decoupling on its +Vs pin.  Still oscillates.  I set it aside for another day.   

Thank you for this!! It may be something I will still try just for the sake of experiment, but good to know.

[TimNJ]

For reference, here is the idea I've been toying with. Relays sound like a disaster but with a sufficiently high resistance kevlin-varley divider, perhaps the contact resistance will not be a problem.

[The Soulman]

Nice project! Word of advice from a newb: simpler=better.
A fully automated kvd would be pretty cool but would require not only expensive resistors but also expensive relays (low resistance, low leakage, low thermal EMF) and would be a major pain to verify without a proper high resolution dmm.

Have a look at the calibrator from Ian Johnston: http://www.ianjohnston.com/index.php/videos/20-video-blog-022-handheld-precision-digital-voltage-source#compare
He uses a (highly linear) 18 bit dac, good enough for 4,1/2 digits possible 5,1/2 dmm's.
Not saying you should go and buy one but just have a look at the design.

[Jay_Diddy_B]

Hi group,
You can use LT1236. The performance is slightly different shunt versus source. You can connect the LT1236 in series, but I would drive them with a constant current source. Similar specs to the AD587.



Regards,

Jay_Diddy_B

[TimNJ]

Nice project! Word of advice from a newb: simpler=better.
A fully automated kvd would be pretty cool but would require not only expensive resistors but also expensive relays (low resistance, low leakage, low thermal EMF) and would be a major pain to verify without a proper high resolution dmm.

Have a look at the calibrator from Ian Johnston: http://www.ianjohnston.com/index.php/videos/20-video-blog-022-handheld-precision-digital-voltage-source#compare
He uses a (highly linear) 18 bit dac, good enough for 4,1/2 digits possible 5,1/2 dmm's.
Not saying you should go and buy one but just have a look at the design.

You are definitely right that keeping it simple reduces the possibility of unintended errors being introduced to the system. I think I kind of have a fascination with the KVD more than anything.

Ah, I remember reading about/watching videos about Ian's calibrator. Cool device! I thought about using a DAC, but I guess in my head, I was thinking, "No DAC could possibly match the performance of a voltage reference plus KVD"...but that probably isn't so. Doing device matching on the silicon level probably isn't that hard compared to the macro/discrete equivalent.

That said, with an 18-bit DAC (maybe 16-bits usable?), you can't calibrate ultra-precise meters, but the guaranteed performance of the DAC might be more important than the hypothetical higher resolution of a 6 or 7 decade KVD. i.e. I might be more confident in a fully tested/spec'd DAC (of lower resolution) than my attempt at an automated KVD. That also said, if I could characterize the KVD, and it turned out pretty good, then maybe it'd be worth something...

Before I scratch this idea completely, would there be any application where an automated KVD (like the one I sketched up) would be beneficial/out perform a DAC based device? Probably not.

Thanks for the insight!

[TimNJ]

Hi group,
You can use LT1236. The performance is slightly different shunt versus source. You can connect the LT1236 in series, but I would drive them with a constant current source. Similar specs to the AD587.



Regards,

Jay_Diddy_B

Thank you for that. Excellent to know

[guenthert]

Ah, I remember reading about/watching videos about Ian's calibrator. Cool device! I thought about using a DAC, but I guess in my head, I was thinking, "No DAC could possibly match the performance of a voltage reference plus KVD"

You might want to look at http://cds.linear.com/docs/en/application-note/an86f.pdf as well as a $30k implementation: https://www.eevblog.com/forum/metrology/2000-post-teardown-and-study-of-fluke-5700a-calibrator/ .

[TimNJ]

Ah, I remember reading about/watching videos about Ian's calibrator. Cool device! I thought about using a DAC, but I guess in my head, I was thinking, "No DAC could possibly match the performance of a voltage reference plus KVD"

You might want to look at http://cds.linear.com/docs/en/application-note/an86f.pdf as well as a $30k implementation: https://www.eevblog.com/forum/metrology/2000-post-teardown-and-study-of-fluke-5700a-calibrator/ .

Thanks! AN86 was definitely an inspiration for this project. I was mostly looking at the 2nd half of the document which focused on the "traditional" KVD implementation. (It seemed easier, at first glance. Not so much anymore.) But, looking at the ADC/DAC implementation now, a lot of the pitfalls of the traditional design seem to be erased. Would not need a boatload of relays and precision resistors. This might be a fairly interesting long-term project to get invested in. That is, basically adapting the AN86 design to make a commercially-viable universal calibrator.

Ian did a great job, so perhaps no reason to reinvent the wheel, but a (properly done) AN86-based design might be able to push even higher levels of accuracy and stability. I do wonder, though, if AN86 has existed for over 15 years, why something similar hasn't made it to market. That is, an economical calibrator. Perhaps there is no market for it at all?

AN86 implements the error correction code on a PIC MCU using assembly. Seems like an low-cost FPGA might be a good candidate in 2017. Will have to get a better grasp on the design before I get ahead of myself though.

Side note, there are DACs like the AD5791 which can just about pull off this level of performance, but they are also about $100/ea. That said, $100 might be justifiable for what you get.

[Kleinstein]

To a certain level, the ADC+DAC solution works. However SD-ADCs  are good and maybe better than many DAC chips, there accuracy is limited. Not that much improvement since the old LTC2400.

For generating a precise DC, there are two more options: one going the AC way and use a precision AC transformer and AC/DC converter with switches. This viable and use in the datron 1281 meter for internal calibration. Also some Russian precision DMMs are supposed to use this way.

A second and probably easier method is using precision PWM. The Fluke 5700 and similar calibrators use it. Especially with a µC to generate the PWM signal and modern parts this looks like a viable way for DIY. There seem to be sum Chinese DIY instructions / implementations this way.

There is not that much market for cheap calibrators. An important part for a calibrator is trust in the company. In some cases a cheap supply in combination with a good meter are used - viable for something like 3 or 4 digit meters.

[TimNJ]

To a certain level, the ADC+DAC solution works. However SD-ADCs  are good and maybe better than many DAC chips, there accuracy is limited. Not that much improvement since the old LTC2400.

For generating a precise DC, there are two more options: one going the AC way and use a precision AC transformer and AC/DC converter with switches. This viable and use in the datron 1281 meter for internal calibration. Also some Russian precision DMMs are supposed to use this way.

A second and probably easier method is using precision PWM. The Fluke 5700 and similar calibrators use it. Especially with a µC to generate the PWM signal and modern parts this looks like a viable way for DIY. There seem to be sum Chinese DIY instructions / implementations this way.

There is not that much market for cheap calibrators. An important part for a calibrator is trust in the company. In some cases a cheap supply in combination with a good meter are used - viable for something like 3 or 4 digit meters.

Thanks for your invaluable knowledge, again. So what is the technical advantage of a PWM based solution? Is high linearity easier to achieve? (That's my guess.) Most microcontrollers I've worked with only have 16-bit PWM resolution, but perhaps there are better that I'm not aware of.

Yes, I agree about the market for cheap calibrators. But I also feel that a well documented product with lots of testing/data to back it might be good enough for some people/labs. Maybe not the big players, though.

[Kleinstein]

The PWM way can be used as a slow, but very linear DAC. It takes a little more than just a basic digital output and a filter, but the circuit is still manageable and does not need high stability parts. It still needs at least one very linear resistor though. So there should be very little drift with temperature and age. There are a few limitations very close the the ends of the range as the on or off time get too short for settling.

The usual µC internal PWM is only 16 Bits, but there are a few ways around this. There are a few µCs with special high resolution PWM that might add something like 4 extra bits and a simulated very high base frequency. However much more than 24 Bits as a pure PWM would produce a rather low frequency and is thus not that practical. One can use a kind of dithering / sigma delta like modulation to reduce the low frequency part and thus simplify filtering a little, and only need a lower HW PWM resolution.

Even if the PWM resolution is only lets say 16 Bits, the linearity can be much better. So it is possible to add a second smaller signal and this way get a higher resolution. The scaling of the second signal only needs to as accurate as the added resolution and is thus not that critical. With the scale of the second signal good to 0.1 % one could add about 9-10 Bits of extra resolution. There are ways to check the scaling so that the µC could adjust the numbers if needed with relatively moderate effort - so the scaling factor would not even need to be long time stable.

[TimNJ]

The PWM way can be used as a slow, but very linear DAC. It takes a little more than just a basic digital output and a filter, but the circuit is still manageable and does not need high stability parts. It still needs at least one very linear resistor though. So there should be very little drift with temperature and age. There are a few limitations very close the the ends of the range as the on or off time get too short for settling.

The usual µC internal PWM is only 16 Bits, but there are a few ways around this. There are a few µCs with special high resolution PWM that might add something like 4 extra bits and a simulated very high base frequency. However much more than 24 Bits as a pure PWM would produce a rather low frequency and is thus not that practical. One can use a kind of dithering / sigma delta like modulation to reduce the low frequency part and thus simplify filtering a little, and only need a lower HW PWM resolution.

Even if the PWM resolution is only lets say 16 Bits, the linearity can be much better. So it is possible to add a second smaller signal and this way get a higher resolution. The scaling of the second signal only needs to as accurate as the added resolution and is thus not that critical. With the scale of the second signal good to 0.1 % one could add about 9-10 Bits of extra resolution. There are ways to check the scaling so that the µC could adjust the numbers if needed with relatively moderate effort - so the scaling factor would not even need to be long time stable.

Again, thank you. I see your point. The LTC1599 spec'd in AN86 has a settling time of 2us. For this purpose, that of course, is pretty unnecessary. I think a 1sec settling time would still be quite reasonable. I see TI and Infineon have an extension for some of their uCs called HRPWM (High Resolution PWM).

I'll have to think about what to do with the PWM signal once its generated. Ideally, we want the PWM signal to control a switch that connects the reference voltage (10V, for ex.) to the filter network. Whatever it is, it will need to turn on/off very quickly to avoid rounding off the rising/falling edges. Given that the filter network will be connected to the input of a buffer, the IR drop across the switch will likely be small, but also predictable. Thus, we can trim the reference voltage to get a perfect 10.000V at the start of the filter network.

Just some thoughts.

Thanks again.

[Andreas]

Hello,

when doing PWM the linearity does not come for free.

- you already mentioned the rise and fall times
- then there is different resistance of the switch when switching the reference or GND
- switch leakage currents
- charge injection from the PWM input to the output.

of course most of them are also temperature dependant.

there is a EDN-cirquit which combines 2 16-bit PWM signals to a 32 Bit PWM output with a fast settling filter.

https://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolution
do not believe all values claimed for the ciruit. I guess it is calculated only theoretically without having built a real cirquit.

some measurements with the famous EDN 32-bit PWM-DAC and different switches attached:
with jelly bean CD4051 I get 120 ppm non linearity. (0.6 mV max)
 (the y-scale is error in mV for a 5V input, the x-scale is the PWM-value (0-65535) applied simultaneous to both pwm signals)

after much trimming and using MAX4051A you can get 3-4 ppm linearity with that cirquit.

with best regards

Andreas