7.5digit diy voltmeter?

[DarkLight]

If the DAC is 10 bits and the ADC ENOB is, say, 22 bits then it would seem that we could get to 7.5 digit precision. There are a lot of details, of course, that need to be correct.

I worked with LTC2400 years ago but never get ENOB above 19.5 bits. 

What has happened to this post?

[RandallMcRee]

If the DAC is 10 bits and the ADC ENOB is, say, 22 bits then it would seem that we could get to 7.5 digit precision. There are a lot of details, of course, that need to be correct.

I worked with LTC2400 for couple of months but never get ENOB above 19.5 bits. 

What has happened to this post?

Nothing! Hey you are reviving it. Although I'm not the OP here I *am* still working on this idea. I feel like a different person posted the stuff above--is there some other Randall McRee? No, I think it more likely that is just a feeling.

So, what I *think* I have learned so far, in no particular order:

Yes, a linear PWM is do-able and I built one that I think will do the job (another thread for that, also moribund)
This is called a differential voltmeter, e.g. Fluke 895, but modernized,
An adc with high linearity and ENOB of 19 bits ought to get you in the 7.5 digit ballpark (overall bits ~ 10+19 = 29)
The devil is in the details; all of those ppm error sources add up so...the devil.

What I am working on now is a 10volt source based on several PX ref LTZ1000 boards.

I acquired a calibrated Keithley 2001 to test all this out. So, yeah, still coming along for me....

Anyone else make any progress? How did you achieve the 19.5 ENOB? How did you verify that?

Irony note: not lost on me that I bought a 7.5 digit meter on the way to making a 7.5 digit meter. Make of that what you will. I try not to judge.

Randall

[TiN]

(overall bits ~ 10+19 = 29)

Somebody got dreaming and carried away, sorry. 32-bit SD ADCs provide ~22-23 ENOB, nothing like "29".
Even if you get magical ADC or DAC with that level of linearity, total system would be limited by lot of other errors to much lower value.

Anyone else make any progress? How did you achieve the 19.5 ENOB? How did you verify that?

You need more linear source, like ramp generator or slow high fidelity sine, or another much better ADC  Or best commercial "DAC" in the world, like Fluke 720A.

Irony note: not lost on me that I bought a 7.5 digit meter on the way to making a 7.5 digit meter. Make of that what you will. I try not to judge.

Mhm, its all downhill since. I wanted to build DIY calibrator, and now I essentially ended up of buying one (second one actually). 

[RandallMcRee]

TiN says

Somebody got dreaming and carried away, sorry. 32-bit SD ADCs provide ~22-23 ENOB, nothing like "29".
Even if you get magical ADC or DAC with that level of linearity, total system would be limited by lot of other errors to much lower value.

Where would we be without our dreams?!
Linear Technology seems to share part of my dream....according to Circuit 1 in AN-78 we have the following table (below).

The nonlinearity is listed as 1+4 = 5 ppm. That is a non-correctable error. So a lower level. If we can control other errors I bet that we will be in the >24 bit range. (Proposal is that Circuit 1 takes the output from the PWM Dac and subtracts the unknown voltage, and presents it to the LTC2400).

AN-78 does not give much detail about how they measured that nonlinearity. It might be the case that those numbers are only true with small common-mode voltages. But I think its worth a shot.

[TiN]

And how exactly did you determine 5ppm INL is ">24 bit range" ? This part does not align with me well

AN86 Appendix C does give enough details on ppm-level INL measurement. Somebody here at EEVBlog bought that very same LTZ reference from the appnote.

[David Hess]

The nonlinearity is listed as 1+4 = 5 ppm. That is a non-correctable error.

The repeatable parabolic INL curve of the LTC2400 can be partially cancelled digitally.

[try]

Hi Tin,

And how exactly did you determine 5ppm INL is ">24 bit range" ? This part does not align with me well

AN86 Appendix C does give enough details on ppm-level INL measurement. Somebody here at EEVBlog bought that very same LTZ reference from the appnote.

That was branadic.

Here is a project that deals with setting up  a measuring and controling system:

https://www.heise.de/ct/projekte/machmit/ctlab/wiki

One of the instruments involved is a DMM that measures voltage and current in DC and AC:

https://www.heise.de/ct/artikel/Messwerkeln-291398.html

This is a project where Andreas got some ideas from.

There are a lot of easy improvements possible to make the device more stable.
I built two of them.

Stop talking and start trying.

Regards
try

[Kleinstein]

Testing the INL is indeed tricky. The AN86 way is nice, but takes quite some instruments - so not a real option for many of us. A full test at all codes is not realistic in the 24 Bit range anyway - it's just not enough time for 10s of millions of test points. There are a few simpler spot tests. Still INL testing is a topic on it's own.

The SD ADCs are not that bad, but they still have a few limitations:
One is that INL is limited to a few ppm's. The LTC2400 is rather good in this respect, but limited to a one polarity. Such an ADC might still be good for an extra test though.
The other limitation is the voltage range, that usually is rather small, so more like +-2 V and +-5 V at best (some Ti converters).
This also applies to the reference - so the 7 V zener reference would need a divider. This makes the long term stability task a little more tricky.
The noise may not be the largest problem. The LTC2400 is not that good here, but something like the LTC2440 is already low noise - though higher INL. The lower noise ADCs tend to use some tricks to lower the noise at the costs of higher INL. Also input buffering can be tricky. Ready made ADC chips are used in the 5 digit and a few (often more lower end) 6 digit meters for a reason.

I am still looking at a more classical multi-slope ADC. Just got some delays. In addition I hoped to learn more from the 6581T "repair" thread  - though there was not that much success, but still some hints and useful thoughts.
So far noise seems to be the least problem . Under good conditions (short and thus no reference noise) I get up to 24 ENOB (500 nV Allan deviation at 20 ms in a +-5 V range), with still some potential to improve on it (higher voltage range, better OP). The difficulty is more like getting a good INL (and test it) and to get good stability (e.g. low drift). It starts with the point that a simple buffer with an OP27 might not be linear enough.

Another point is finding a suitable input stage: the simple 1 stage amplifier and switching used in the HP meters like 3456, 3458 might have quite some input bias if used with a 1 PLC AZ mode - at least it gets challenging to make the input low bias. One point learned from the 6581T is that switching the input is not that easy. So I currently favor a 2 stage design like in the Keithley 200x.
For a SD-ADC based DVM, I would consider is different input configuration that takes advantage of usually having a differential input. 

[Andreas]

INL adjustment on a LTC2400 is rather easy.
All you need is
- a stable reference (LM399#3 in the cardbox in the background)
- a (short time) temperature stable resistor string (right) (here with 0.1% 25 ppm/K resistors amplified up to 10V)
- a (high impedant) buffer amplifier (in the middle)
- a LTC2400 (left)
- a plastic pincer (blue) (to avoid thermal voltages when changeing the resistor taps)

you simply have to rely on the fact that the lower part and the upper part of the resistor string give the total resistor string voltage.
And at the LTC2400 you can model a parabolic curve for the INL which makes it even more easy.

For measuring of voltages larger than 5V I use additionally a LTC1043 precision divider.
So usually I make 2 measurements for the INL: one with LTC1043 (buffered on both sides) and one without LTC1043.

with best regards

Andreas

[hwj-d]

Stop talking and start trying.

Certainly, that's not to TiN ...   
(edit, he always tries everything)

[DarkLight]

Anyone else make any progress? How did you achieve the 19.5 ENOB? How did you verify that?

I achieved the 19.5 by using LT1236A, separated battery power supply  and a lot of try and error and verify it using Arduino and some equations to calculate ENOB . Here I attached the schematic.
I have also uploaded a video on youtube, but the video was recorded when I got 18.5 Enob.  https://goo.gl/6C9bWf

[Gyro]

INL adjustment on a LTC2400 is rather easy.
All you need is
- a stable reference (LM399#3 in the cardbox in the background)
- a (short time) temperature stable resistor string (right) (here with 0.1% 25 ppm/K resistors amplified up to 10V)
- a (high impedant) buffer amplifier (in the middle)
- a LTC2400 (left)
- a plastic pincer (blue) (to avoid thermal voltages when changeing the resistor taps)

you simply have to rely on the fact that the lower part and the upper part of the resistor string give the total resistor string voltage.
And at the LTC2400 you can model a parabolic curve for the INL which makes it even more easy.

For measuring of voltages larger than 5V I use additionally a LTC1043 precision divider.
So usually I make 2 measurements for the INL: one with LTC1043 (buffered on both sides) and one without LTC1043.

with best regards

Andreas

Just for my curiosity, but why would you implement an LTC2400 setup with single ground pin shared by input, reference, supply current and digital interface returns, when you could use an LTC2410 with differential input and reference, multiple ground pins etc.

The LTC2410 is actually cheaper than the 2400 (~£7.60 vs £9.70) from RS in the UK, not sure about other places. In fact why did LT bother to make the LTC2400? It just seems way too much of a compromise to squeeze a 24 bit ADC into such a low pin count SO8 package.

As I say, really just for my curiosity, not a criticism.

[hwj-d]

Then I would probably prefer an ADS1256.

[David Hess]

It starts with the point that a simple buffer with an OP27 might not be linear enough.

This is probably nothing new to you Kleinstein but I will post for others who might not be as familiar with precision design.  There are various things which should be done or at least avoided to improve the linearity of a precision operational amplifier:

1. Watch out for the temperature coefficient of resistance of the feedback network.  When configured for gain, the difference in heating of the input and feedback resistors can change the gain enough to matter at different output voltages.  This conflicts with point 3 below if a high impedance divider is used.

2. Unload the operational amplifier's output to prevent self heating.  Thermal feedback from the output transistors to the input transistors limits open loop gain among other things.

3. Watch out for non-linear changes in input bias current with common mode voltage. This is especially bad with some FET input operational amplifiers.  This conflicts with point 1 above if a low impedance feedback divider is used.

4. Errors due to limited common mode rejection can be lowered by using chopper stabilization or bootstrapping.  Both are common in the best bench voltmeters.

Weren't early OP-07s (and OP-05s?) known for low open loop gain yielding poor linearity?

There are some test circuits which can be used to measure and display the non-linearity of an operational amplifier for evaluation:

http://www.ti.com/lit/an/snaa047a/snaa047a.pdf
http://www.introni.it/pdf/Bob%20Pease%20Lab%20Notes%20Part%207.pdf
http://application-notes.digchip.com/006/6-8872.pdf
http://www.ti.com/lit/an/snoa737/snoa737.pdf

[Andreas]

As I say, really just for my curiosity, not a criticism.

The answer is already here:

https://www.eevblog.com/forum/metrology/7-5digit-diy-voltmeter/msg1395104/#msg1395104

The main advantages are the overrange (how do you adjust zero and full scale with a ADC that clips hard the values at zero or full scale), the very low T.C. and a predictable INL.

with best regards

Andreas

[Gyro]

Many thanks Andreas,

I hadn't spotted the extended underrange / overrange conversion capability on the LTC2400. Without going through the datasheets in detail, I had assumed that the 2400 and 2410 might be different bond-out versions of the same die, obviously not. It's a shame they didn't include it on the 2410 for ground referred input and reference inputs.

Thanks,

Chris

[e61_phil]

The 2410 is a "real" differential ADC. The -IN shouldn't be tied to ground. Therefore, the 2410 has even more "underrange" than the 2400 and you can easily measure zero.

For example: In a 5V system you can tie -IN to 2.5V. The +IN can now move from 0V (-Vref/2) to 5V (+Vref/2).

The INL specification from the 2410 is better than the spec of the 2400. But I don't know what that mean in practice.

[Kleinstein]

For the differential SD ADCs the INL and other specs tend to be for a true differential signal: the negative input being close the to inverted version of the positive input. So having one input tied to a fixed voltage can increase the INL error. In addition it might limit the range (can be relevant with the ADC1256). For a DVM circuit measuring an isolated voltage it is possible to get this by having the common terminal not tied to ground or a fixed level, but to the negative ADC input and drive this to the inverted positive input.

The LTC2400 INL is mainly a simple square contribution that could be compensated for, at least to a large part. So there is a reasonable way to reduce the INL.  For the LTC2410 the shape is more complicated and thus compensation more difficult.

[Andreas]

Hello Phil,

yes of course.

But you have to pay attention that the 2.5V are
- stable (so not to spoil the ADC specs for offset and full scale drift).
- slightly higher than 2.5V (otherwise you cannot reach the 0V and the 5V end values)

Unfortunately a LTC1043 divider delivers usually slightly lower than 2:1 ratio.

with best regards

Andreas

[Echo88]

Since this is a fitting thread for some questions, i ask them here:

Are there any reliable/proven infos on the long term stability (at least capacitors age) of the LTC1043 if used as a voltage divider/multiplier, for example to generate  7.2V / 3 * 2 = 4.8V as reference for a LTC2400?
Are there any very low INL and >= 24Bit-ADC-modules like the Thaler ADC180 or the PREMA 5610E which can be bought at the moment from normal persons? Maybe some guy already rolled their own Multi-Slope-ADC as Open Source and i didnt find it?
Can the INL of every ADC reliable be corrected (suppose ADC is in oven, temperature variation not relevant) or does it drift with time?

[Kleinstein]

The error of the LTC1043 or similar charge pump divider is rather small and constant over time. It does not depend very much on the drift of the capacitors it is more like parasitic capacitance and charge injection that give a small error. So this would not be the largest point to worry about.

There are a few ADC chips with low noise (e.g. LTC2440, LTC2368-24 ) - for some there are evaluation board available. With sufficient averaging one can get >24 bits ENOB for rates reasonable for a DMM.

Correction of the INL is difficult in general. It can be possible / practical for a smooth function type like with the LTC2400, where there is an extra x² or x³ contribution, but not very practical with a more complicated function like with many of the faster SD ADCs. It may also need quite some effort to measure it to correct it - this gets much easier if one knows it is predominantly a certain simple function and thus a simple test is enough to estimate 1 or 2 parameters.

I am currently on a DIY multi-slope converter (HW somewhat in between the HP34401 and Keithley 2000/2010), and a plan to release it open source. So far it looks very good for the noise (good enough for 7 digits), but no really stringent INL test so far, as its breadboard only so far.

[EmmanuelFaure]

Are there any very low INL and >= 24Bit-ADC-modules like the Thaler ADC180 or the PREMA 5610E which can be bought at the moment from normal persons?

Another one : AD1175K, 22 bits, made by AD in the 80's but obsolete now.
Clic : http://www.analog.com/media/en/technical-documentation/obsolete-data-sheets/1787587AD1175.pdf

[hwj-d]

Btw, is somewhere described how dc-acal of the 3458a works in detail?

[Kleinstein]

There is likely some description on ACAL work in the manual and also in the HP Journal article on the 3458.

For the ADT 6581, there is quite a detailed analysis on the ACAl operation. A different meter, but similar in respect to ACAL for DC.
The ACAL part is for most parts quite straight forward, though there might be still some small tricky points with waiting times and maybe self heating for the shunts (ACAL tends to use the shunt at a higher current than normal operation).

The difficult part of ACAL is more like the AC part.

[Echo88]

Thanks Kleinstein! Im sure there would be many people interested in testing your OSHW-ADC.
Regarding the proposed LTC2440: If i were to connect a floating voltage to the input, which is variable between +-Vref/2, then the INL should behave like the "Integral Nonlinearity fOUT = 6.875Hz" on page 6 of the datasheet, while i wouldnt worry about the common-mode-voltage since my input voltage is floating?

http://www.analog.com/media/en/technical-documentation/data-sheets/2440fe.pdf

I still have my LTC2508-32-Evalboard, which should still work and should have a little less noise compared to the mentioned LTC2368-24. I will order the LTC2440-Evalboard and also a LTC6655 to solder it on the board (since the LTC2508-32-Board also uses one).
Throw both boards in an temp-regulated-oven (borrowing knowledge from the oven-thread  ), power it with quit supplies and log the data with Python.