7.5digit diy voltmeter?

[Bootalito]

I was thinking of embarking on a voyage: constructing an open source DIY 7.5digit volt meter.  Now I know doing this is more than just buying highly speced parts and slapping them together, but it doesn't seem out of the realm of possibility to do the following:
1. Construct an xdev's LTZ1000 reference, and burn it in for 1000hours
2. divide the reference in half(or something) using a precision resistor network and feed this into a 24ENOB ADC like AD71772 (has built-in rail to rail op amps).  Its not like a can design my own custom multi-slope integrator....
4. Feed test voltage in through another precision resistor network (so only a single range volt meter), or not...make it like a 5V max volt meter or something...
5. Add some input protection: PTC, MOV, maybe a Zener in case someone feeds in to much voltage.
6. Use a linear power supply with a properly isolated digital ground and analog ground
7. Use some sort of uC, a decent size LCD, a couple of rotary encoders, a couple of pushbuttons
8. Bob's your uncle!

Of course I would expect a long process with experimentation, and many board revs, but with the right engineering a 7.5digit volt meter, or at the very worst a 6.5digit noiseless volt meter.  I couldn't find anywhere where someone has taken this on, or perhaps I'm wrong?

[The Soulman]

I think you meant AD7172?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD7172-2.pdf

24 noise free bits at 5 SPS

In theory it can reliable produce 16777216 counts or 7,5 digits, accuracy and linearity at that level are a entirely different thing.

Edit: Scullcom hobby's on youtube did a similar project, was going to link it here but youtube ain't co-operating. 

[james_s]

It sounds like an interesting project if your primary goal is to stretch your engineering abilities and pull off a unique challenge. On the other hand if you're trying to reduce the cost of 7.5 digit meters I don't think that's going to happen, frankly I'd be surprised if you can get all the parts for less than what a good used commercial meter goes for.

[Bootalito]

@Soulman
I meant the AD7177-2 with the 24.7ENOB @ 5SPS http://www.analog.com/media/en/technical-documentation/data-sheets/AD7177-2.pdf
I've watched all of Scully's videos, and this project would kind of be an extension of that. It doesn't look like he ever attained the precision, accuracy, he was shooting for, nor did he complete any long term stability testing.  Greg Christienson improved upon the design here: https://www.barbouri.com/2016/05/26/millivolt-meter/#comment-523, however upon close inspection of his PCB layout I think he may have made a few errors like not including the vREF and ADC in AGND with a star ground at the ADC, not providing a direct current path back to ground for the VREF and ADC (which would probably be fixed if they were put in AGND), placing DGND under the VREF and ADC, using a common LM7805 instead of a low noise linear reg, placing the linear reg close to the ADC and VREF (heat) , placing a ground plane right next to power traces (capacitance), and placing some bypass caps down an "alley way" trace away from pins.

@james_s

Yes this project would be solely as a new challenge for myself.  It would force me to upgrade my equipment so I can take the appropriate measurements (my wife is going to kill me...), I'd have to develop all kinds of testings procedures to collect accurate repeatable data, etc.  I'm not even an electrical engineer (Chemical Eng by degree, Control Systems Eng / programmer by trade) or have any formal electronics training.  Even if I can only pull of a decent 5.5digit meter the experience would be invaluable.

[ez24]

Thanks for your info and links.  I am trying to determine if there was alien interference into the electronics field in the 1990's.  I had given up until I found a link to a link in your post.  So I am working with what I can find now.     There were power supplies that were very advance and in 20 years they have completely disappeared (the people).  There was a photo in the links that said it was them but it was a government building that has rocket shape designs on it.  I hope they find out about this message and invite me to their planet to stop me from exposing them because I need a vacation.

[Bootalito]

Thanks for your info and links.  I am trying to determine is there was alien interference into the electronics field in the 1990's.  I had given up until I found a link to a link in your post.  So I am working with what I can find now.     There were power supplies that were very advance and in 20 years they have completely disappeared.  There was a photo in the links that said it was them but it was a government building that has rocket shape designs on it.  I hope they find out about this message and invite me to their planet to stop me from exposing them.

I'm sorry...what now?

[David Hess]

The AD7172 by itself is not going to support high input impedance dividers so you are also going to require an impedance converter with 7.5 digit accuracy.  That likely means bootstrapping and input bias current cancellation if not some capability to do automatic zeroing.

[beanflying]

I have been rewatching the Bob Pease videos again. Well worth it if you haven't or even worth watching again. One on noise too.

[RandallMcRee]

Sounds interesting.
I would like to do something similar. My thinking is to use a 'standard' A/D evaluation board from either TI, AD or LT.  So for the 7177-2 you could use the http://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/EVAL-AD7177-2.html
If your project goes with, say, the LT25000-32, then this
http://www.linear.com/solutions/7783 (but you would need to use one of their companion boards in addition)
http://cds.linear.com/docs/en/video/LTC2500-32_Thoren_v2.mp4

The purpose of deciding on an eval board is simply to leverage the ability to write software for the A/D side of things, as well as removing uncertainty around some of the precision analog engineering. Plenty left, of course. What are your thoughts on leveraging such an eval board?

At the end, I think you will still need a calibration step in order to meet your goals, btw.

[David Hess]

Neither of the evaluation boards are intended for something like a 7.5 digit (or better) DC voltmeter application.

[Vgkid]

I have been rewatching the Bob Pease videos again. Well worth it if you haven't or even worth watching again. One on noise too.

*snip*

I have long been looking for that in print(online) , but have never found it.

[beanflying]

I haven't seen it is print. Not sure how many of the old videos are available by % of what was ever done but lots to be gained from them especially now as a lot of 'doesn't matter we can fix it in firmware/software' seems to happen to often.

I do love the rats nest prototyping 

[branadic]

Hi,

looking at the datasheet of AD7177-2 I can see a typical INL with all input buffers disabled of typical ±1 ppm of FSR and max. ±3.5 ppm of FSR, with an unpredictable shape of error curve. It will be a challenge if not impossible to linearize the beast to perform as a comparing 7.5 digit instruments.

If you have a look on PRI 5610 26 bit ADC once made by PREMA you find:

PRI 5610 E:   typ. 0,05 ppm,   max. 0,08 ppm
PRI 5610 F:   typ. 0,1 ppm,   max. 0,2 ppm
PRI 5610 G:   typ. 0,2 ppm,   max. 0,5 ppm

unfortunely used in their 7.5 and 8.5 digit multimeters.

The good old LTC2400 datasheet quotes INL:

VREF = 2.5V:   typ. 2 ppm,   max. 10 ppm of VREF
VREF = 5V:   typ. 4ppm,   max. 15 ppm of VREF

with a predictable error curve that can be calibrated to <1ppm with only a few points. You can also have a look on this board:

OSHW - 24bit ADC measurement system for voltage references

There have been several attempts to build up a voltmeter in the past, so maybe you want to start reading the Open Source Hardware section first:

Open Source Multimeter

and don't forget what you are aiming for:

7.5 digit accuracy means 0.05ppm linearity while ±1ppm linearity error means something like 5.5 digit accuracy. The rest is for the dumpster.



-branadic-

[mycroft]

Is there a follow up (video or text) for this video? The circuit looks very interesting!

[BU508A]

This is the datasheet of the PRI 5610 ADC made by Prema.

[Kleinstein]

There usually is no need to get INL down all the way to the resolution limits. There is still quite some use of resolution better than the INL. Still a good meter should give you a good INL. For most of the part INL is more difficult than noise - thus it can still make sense to have a 7 digit resolution with an 1 ppm INL error.

Still the integrates ADCs tend to work on a small scale, like +-2.5 V or so. This is a little unconvenient with the better voltage refs at 7 V or 10 V. So if  would be good for a meter to have a well working (low INL and high impedance) range that includes 10 V. This get difficult with the integrated ADCs.

[Kleinstein]

Getting the noise down is not that difficult anymore with modern parts. One big advantage with noise is that one can pretty much calculate up front how much noise one can expect from a circuit. The noise specs on the integrated ADCs are somewhat limited, but some are rather good and would be sufficient for 7.5 digits.

With INL it is not that easy to do an upfront calculation, as at the PPM level there are tiny oddities like self heating, dynamic loading of the reference, nonlinearity of resistors and caps can come into play. Also supply "noise" can come into play - so something like power decoupling can come into play. Also OPs often only have CMRR specs in the 120 dB range (or less) - this could cause 1 ppm INL already, though its usually less. Another factor can be switches - those CMOS switches on resistance is not perfectly linear.  It is hard to tell how the contributions to INL will add up together as the specs of the parts are usually not that specific. INL is just more difficult to describe than noise. At best one gets typical curves, but who knows how typical they are. Testing the INL to sup ppm level is also rather difficult and slow.

Compensation for non-linearities is possible in some cases, if this is a kind of simple type, like a 3rd power contribution one might get from self heating. However it still needs a kind of reference and extra adjustment measurements - so this can usually only be done for a limited number of points and temperatures. Especially the lower noise SD ADC chips tend to have an INL that is more of the odd shape and thus essentially impossible to correct - this is because they kind of need to go beyond the simple 1 bit 2nd order SD concept.

If you kind of copy a multislope ADC design of lets say the Keithley 2000 or 2001 or HP34401, getting the noise lower that the original should be relatively easy with modern parts. The difficult part is getting it really linear - that is better than 10 ppm INL. That is the part where the magic is build in.  Most of those odd badges / changes and tweaks in the Datron 1281 are for linearity, not for noise reasons.

[Bootalito]

Amazing discussion so far. Thanks for everyone's insights. I really hadn't considered INL, as I only learned of this last night! I suppose this is why custom multislope adcs are used.
Since I don't have the equipment or the expertise yet, I think the only path that would get me even close would be to design and fully characterize successively better volt meters(3.5, then 4.5, 5.5,etc).  This of course will maximize the Learning/cost ratio and see how far down the rabbit hole I'm willing to tolerate. I mean... To even test a 7.5d meter I'd need an calibrated 8.5, and i can't justify buying one right now(to my better half.. I can justify anything to me, heh)

I do like the idea of using evaluate boards. At a minimum to get started and to see how things "should" be designed. And at most to be the backbone of a good 5.5d or even 6.5d design. Maybe?

I've also been thinking of starting a YouTube channel for the main purpose of sharing my industrial control knowledge with other Automation engineers(pid loops, difference between diff and se inputs, etc). A lot Automation engineers are operating under the "someone more senior told me to do it this way 10 years ago" and really don't know what they are doing. So I think I'd try to document every step of this journey in the fashion of Scullcoms channel (project based tutorials sprinkled with thoery/design explanations)

[Magnificent Bastard]

I don't think you are going to be able to buy an off-the-shelf ("we do it all for you") ADC chip that has 24-bit precision.  Note that accuracy != precision != resolution.  Your ADC is not going to be more accurate than the reference.  At best, this parameter is limited to somewhere around 1ppm/year.  The precision is the INL spec; this means that each bit has meaning, and is repeatable (and/or predictable).  You can have 1000 bits of resolution, but if those bits are not repeatable and/or predictable (and linear) from measurement to measurement, then they are a waste of time and money, and you are just fooling yourself.  Noise can only be filtered out if it is Gaussian. For 7.5-digits, what you are looking for is >= 25-bits of precision (which is the INL spec, even if you need "help" from the firmware to achieve that).

You *CAN* get the necessary precision for 7.5-digits (or even 8.5 digits, but that is difficult for a hobbyist), by using discrete parts (precision op-amps, low-noise dual FETs, high stability resistor ratio dividers, etc.)  Probably, the best architecture (these days) is "Delta-Sigma", but implemented with high performance parts, using multiple integrators and a multi-bit feedback DAC-- but not with a monolithic IC.

Probably, a good "first try" would be a 6.5-digit ADC (with < 1-LSB INL)-- if you can get to that point, then you will know what needs to be improved to get to 7.5-digits (with < 1-LSB INL).  Going from 6.5-digits to 7.5-digits will take at least an order of magnitude of effort, and maybe even an order of magnitude of cost.  8.5-digits (with <= 1-LSB INL) would (probably) require some custom parts as well as some highly selected parts in order for the last digit to have any meaning (other than as a marketing tool).  Going from 7.5-digits to 8.5-digits would require another order of magnitude of effort and BOM cost.  Also, at 8.5-digits, you are getting into the realm of Black Magic and Fairy Dust.  That's why I said that 8.5-digits is probably not something an average hobbyist can achieve.  I'm pretty good with designing analog electronics, but I don't think that even MY "Analog Fu" is powerful enough to pull off a good 8.5-digit ADC design all by myself.

There is a possibility of working on a "community designed" 8.5-digit ADC (with DMM front end)-- if the top talented engineers here on this forum would work together, then this might just be achievable.  It would certainly be a lofty goal to strive for.  I've noticed that there are a lot of knowledgeable engineers here, and that is a lot of "Engineering Fu" in one place-- maybe even more than someplace like Keysight or Fluke.  The problem is, getting 3-dozen (or so) highly-talented engineers (usually with big egos) to work together is a lot like "herding cats"; Good Luck with that!

[tszaboo]

2. divide the reference in half(or something) using a precision resistor network and feed this into a 24ENOB ADC like AD71772 (has built-in rail to rail op amps).  Its not like a can design my own custom multi-slope integrator....
4. Feed test voltage in through another precision resistor network (so only a single range volt meter), or not...make it like a 5V max volt meter or something...

You missed step 3. But I can help with that:
3. Realize that said resistor network doesn't exist.

[Andreas]

Hello,

I was thinking of embarking on a voyage: constructing an open source DIY 7.5digit volt meter.

That´s what I am trying (hard) since 2008.

4. Feed test voltage in through another precision resistor network

That what you need in precision so that it does not spoil the performance of your reference is not available on the hobby/industrial market. -> use better a LTC1043 instead. (ok it will increase perhaps some noise but not spoil the performance of your reference).

6. Use a linear power supply with a properly isolated digital ground and analog ground

do not forget a shield between primary and secondary side of the transformer windings.
and in any case you also need at least photocouplers between floating side and ground referenced side of the instrument.
(but even they have around 0.25pF each which can couple some noise into the ADC).

Of course I would expect a long process with experimentation, and many board revs,

so true words. At least you will learn much about parasitic behaviour of of the shelf components. Including humidity sensitivity by swelling of PCB material.

with best regards

Andreas

[Bootalito]

2. divide the reference in half(or something) using a precision resistor network and feed this into a 24ENOB ADC like AD71772 (has built-in rail to rail op amps).  Its not like a can design my own custom multi-slope integrator....
4. Feed test voltage in through another precision resistor network (so only a single range volt meter), or not...make it like a 5V max volt meter or something...

You missed step 3. But I can help with that:
3. Realize that said resistor network doesn't exist.

Oh... Well shoot. Wouldn't a 5ppm tolerance network with 0.5ppm TC be good enough with proper software calibration?

[Andreas]

3. Realize that said resistor network doesn't exist.

you were faster: but in reality the hope that it exists will never die. 
So that will be the last step.

With best regards

Andreas

[Bootalito]

Hello,

I was thinking of embarking on a voyage: constructing an open source DIY 7.5digit volt meter.

That´s what I am trying (hard) since 2008.

4. Feed test voltage in through another precision resistor network

That what you need in precision so that it does not spoil the performance of your reference is not available on the hobby/industrial market. -> use better a LTC1043 instead. (ok it will increase perhaps some noise but not spoil the performance of your reference).

6. Use a linear power supply with a properly isolated digital ground and analog ground

do not forget a shield between primary and secondary side of the transformer windings.
and in any case you also need at least photocouplers between floating side and ground referenced side of the instrument.
(but even they have around 0.25pF each which can couple some noise into the ADC).

Of course I would expect a long process with experimentation, and many board revs,

so true words. At least you will learn much about parasitic behaviour of of the shelf components. Including humidity sensitivity by swelling of PCB material.

with best regards

Andreas

Well thanks once again Andreas I've never even heard of a switched capacitor building block....

[Andreas]

Oh... Well shoot. Wouldn't a 5ppm tolerance network with 0.5ppm TC be good enough with proper software calibration?

How do you calibrate
- humidity sensitivity (very long time constant of several days)
- hysteresis of several ppms

in my resistor measurements I have found generally:
Resistors with a relative low T.C. have a relative large hysteresis.

with best regards

Andreas