7.5digit diy voltmeter?

[branadic]

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

Hope dies last, but it dies

-branadic-

[Edwin G. Pettis]

Andreas,

Resistor TCR has absolutely no bearing on hysteresis, that is wholly a matter of construction and residual stress left after manufacture.  It is present to some degree in all resistors and tends to reduce with time even in resistors with higher hysteresis.  It all depends on how the resistor is made, what materials and what processing was used.  TCR is the inherent parameter of the wire when it is made, the process of making a resistor can certainly affect the 'apparent' TCR of the resistor but normal operating temperatures, even +125°C absolutely will not affect the inherent TCR of the alloy Evanohm.  Now if you're talking about Manganin, that is a very sensitive alloy, sensitive even to barometric pressure and usually takes quite some time to relax after manufacture.  All precision WW resistors will have at least a little bit of residual hysteresis after manufacture, while additional processing can remove more of it, there will always be some amount lingering for awhile, it will fully relax after some time.  Resistor standard grade resistors have not only gone through additional processing but they have also had additional time on the shelf to relax.  Yes, some resistors just might not to fully relax even after some time has passed, it happens.  Even film/foils have residuals that might not fully relax or go away with time, we're talking some pretty minute effects here.....patience is the best teller of quality, very good resistors will settle down to very small drift over time, others not so much.  Precision wire wound resistors are hand made after all and even the machine made Vishays aren't perfect either.

[Kleinstein]

When using a SD ADC chip which needs a reference voltage in the 2.5 to 5 V range, it might be easier to directly start with a reference chip that is made for this voltage range, e.g. LTC6655 or similar.  For long term checks one might include an extra LM399 that is than measured in the corresponding range if needed.

For a DMM quite a lot can depend on the quality of the resistors used.

There are difference limiting factors for a DMM. Usually noise sets the resolution,  INL, calibration and gain/reference drift set the accuracy and drift limits precision.  These limits can be at different levels, though with the commercial instruments they are often in a reasonable relation, though there are also differences. Some are good at low noise and other offer good INL despite of relatively high noise (e.g. Keithley 2001).

There is still some sense in having resolution much better than accuracy (e.g. due to calibration limits or higher INL). It is quite common that the INL and calibration / stability is not as good as the resolution. So INL is important, but no need to get INL to 1 ppm for a 6 digit resolution. Especially if you know about the limitations even a 10 ppm INL with 7 digits of resolution can be OK.

[tszaboo]

I have time to elaborate now. The best resistors networks will only give you 1-2ppm tracking. Even if they advertize 0.0x ppm tracking, those are typical values. Mean tracking, with +/-2ppm maximum values.
http://www.vishaypg.com/foil-resistors/voltage-dividers-networks/

This means, you can potentially select a resistor network which works for the application, but the selection is very difficult (you need to measure in the PPM region in the first place to select it) and costly. And you need to buffer this most of the time, adding extra noise and errors. And your end result is comparable in performance to a series bandgap voltage reference.

The reason it is not done is not because the lack of interest. If it would be so simple, as getting an LTZ1000 and dividing it by two, everyone would be doing that.

That being said, it is possible to improve a system with a heated buried zener reference, and bring it above the capabilities of the bandgap voltage reference.
Or it is useful to make systems, where resolution INL, DNL is higher than a 6.5 digit multimeter. Ratiometric measurements for example.

[Andreas]

the LT2400 which you and branadic used for TC measurement works, at this moment, approximately what average resolution could those produce now (with your improvements)? 5.5D ?

Hello,

that depends.
Mainly on the effort which you put into each single device.
And the comparison to any x.x digit device is difficult.
You should compare it more to a old Solartron or similar
with long integration times on high resolution
than with a modern multi-slope instrument.

If you just build it and do full scale adjustment it is actually something like a 5.5 digit instrument.
Noise for a single measurement is relatively high:
- 10uVpp or 1.8uV RMS in 0..5V range corresponding to 20uVpp 3.6 uV RMS in 0..10V range with a LTC1043 divider.
With averaging over one minute you go down to around 0.5uV RMS for a 7V stable reference
 in 10V range which is about factor 5 above a HP3458A.

What I am doing is the following:
Buy plenty of good voltage references (AD586LQ).
Select them for T.C. , hysteresis and popcorn noise.
So from 10 references you get (in average) around 2 which are suitable:
    (T.C. < 1 ppm/K (better 0.3 ppm/K), hyst < 1ppm, popcorn < 2uVpp)

Put the best of them onto ADCs which have a NTC for temperature correction.
Adjust them for T.C. (3rd order correction), INL, Gain.
The newer devices I also adjust for offset and offset drift (then paired with a LTC1043 divider cirquit).
Adjustment takes around 4-6 week-ends for each device.

Then you have simply to wait the run-in phase (switched on) until the AD586 has stabilized.
(typically 5000 hours)
You can shorten this phase partly by loading the AD586 with a 15 mA current cyclically.
(30 minutes on during  2 hours cycle).
After that you typicaly have to adjust the gain. (T.C. and INL normally only need to be checked).

What you get (typically):
- Noise 0.5uV eff @10V (1 minute integration time)
  (about factor 2 above a 1 minute average of a 6.5 digit DMM)
- INL < 1ppm (that what is specced on many 6.5 to 7.5 digit DMMs)
- Long term stability about 1-2 ppm/year (also comparable to well aged LM399 based instruments).
- Excellent temperature stability which is essential in my lab.
  (from 18-33 deg C max 1 ppm drift)
  That is why I cannot use a 6.5 DMM in 10V range for my T.C. measurements.
  The drift over one day is usual several PPMs on a K2000 or 34401A with my lab conditions.
- even better stability you get for ratiometric measurements.

But nothing comes for free.
A 6.5 DMM might be cheaper when counting all the gear you need to adjust a ADC.
And even then for exact absolute measurements there is no Auto-Zero implemented up to now.
So the 2-6uV offset you have to measure before/after each measurement.

But all in all with long integration times you have something like a 6.5 digit instrument with
some features (T.C.) being even much better.

With best regards

Andreas

[e61_phil]

- INL < 1ppm (that what is specced on many 6.5 to 7.5 digit DMMs)

I don't know the K2000, but the 34401A is normally way better than ~1ppm INL, even if it is specced worse.

As always, it is a question of the application. If you want to monitor some drift, then a high resolution may be more important than a very good INL. If you want to make decade transfers, than 1ppm isn't that great (10ppm error for 1:10).

[quarks]

many years ago I was interested to build a voltmeter from c't magazine (c't 02/2008, page 170).
It was part of a DIY project called "c't-lab".

Unfortunately it is in German but with schematics, parts list and details about design decissions.
Maybe this could be a helpfull starting point.

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

[Andreas]

many years ago I was interested to build a voltmeter from c't magazine (c't 02/2008, page 170).
It was part of a DIY project called "c't-lab".

Unfortunately it is in German but with schematics, parts list and details about design decissions.
Maybe this could be a helpfull starting point.

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

Hello,

a good starting point: yes. (it is a 5.5 digit design).
I got also some inspiration of the design (especially the R/C filter directly at the ADC).
But there are some mis-conceptions.

Input voltage divider built from standard 0.1% metal film resistors instead of a much better ready made divider.
2.5V band gap reference and +/- 2.5V measurement range, which increase linearity, but increase the relative noise by a factor 4.
A LM399 reference is option but uses a trim-pot and metal film resistors which spoil the good T.C.

with best regards

Andreas

[Andreas]

Hello,

here the reason why I prefer my ADCs in my unstable "lab" conditions over a 6.5 digit DMM:

During T.C. measurement of a (very stable) LTZ-device the room temperature changed somewhat.
The dips are due to a open window: perhaps I should not have done this but I would have a larger "end" temperature at the evening without that.

My ADCs (green + red) are noisy but very stable over room temperature.
ADC16 is more stable since it is additionally in a temperature controlled environment.

The DMMs show a relative large influence from room temperature.
The K2000 also seems to have a not sufficient warm up phase but is then more stable than the HP.

with best regards

Andreas

[ramon]

LTC2400 datasheet and every application note or pdf document I have read clearly states that it is intented for a 6-digit DVM application.

So only 5.5? what is needed to reach the 6 digit for LTC2400?

[e61_phil]

LTC2400 datasheet and every application note or pdf document I have read clearly states that it is intented for a 6-digit DVM application.

So only 5.5? what is needed to reach the 6 digit for LTC2400?

Linearity

[Kleinstein]

The INL of the LTC2400 is rather high to directly use it for a 6 digit DVM. However the INL curve seem to be rather predictable, and after correcting that predictable part the linearity should be good enough for a 6 digit DVM.  The noise of the LTC2400 is also relatively high - so it kind of makes it a slow 6 digit ADC.

Not all of the DVMs are equal in that respect - some are rather noisy too (e.g. HP3457, old Solartrons and Keithley 2001 - despite of claiming 7 digits).
 
Another point that make the use of the LTC2400 a little tricky is that the input range is positive only with only a minimal negative range. So real life the LTC2400 is more suitable for a 5 digit DMM or maybe a 5-6 digit display for a measurement that can be positive only (e.g. at a lab supply or maybe balance). The LTC2400 is already rather old and at it's time a real option for an DMM.  The Hameg 8012 used the LTC2400 for a 4 3/4 digit DMM, so even a little less than 5 digits. It is still a step forward from a more traditional ICL7135 4.5-digit dual slope converter.

[Andreas]

The LTC2400 is already rather old and at it's time a real option for an DMM.

Do you have a better modern ADC which has those features of the LTC2400?

- some mV overrange so that you can do full scale and zero adjustments without clipping?
- predictible INL or alternatively INL well below 1 ppm.
- the very low offset and full scale error
- the extremely low offset and full scale error drift (0.01 or 0.02 ppm/K)
- good hand solderability (so for me TSSOP is already difficult)
additionally on my wish list is of course a factor 10 lower noise and a +/-10V range input with some % over range.

but I fear with all those features the LTC2400 still has its market.

With best regards

Andreas

[ap]

Do you have a better modern ADC which has those features of the LTC2400?

e.g the LTC2508-32

[branadic]

e.g the LTC2508-32

Predictible INL or alternatively INL well below 1 ppm?

-branadic-

[Kleinstein]

At least the "typical" INL does not look that bad for the 2508. There seems to a small predictable part and a something like 0.5 ppm if more or less random variations.  The curve on the right seem to be the error including the common mode error, e.g. driving both the + and - input to different common mode voltages. So if you can avoid an uncontrolled common mode voltage, this does not look that bad.

For a DMM the unipolar range of the LTC2400 is kind of a problem and would need an added offset, or switching depending on the sign of the input voltage. This can easily degrade the drift specs or the INL or both.  For the more normal +-xx V range I would prefer an ADC with bipolar input, like the LTC2410 or ADS1234. The ADC1210 would be even available in a package suitable for easy hand soldering (SO18)  - though kind of expensive. The INL specs are not that bad if you only use 80% of the range and keep the rest for over range with degraded INL.
Some of the Ti converters work up to +-5 V and thus a useful +-4 V range with good INL.

For a DMM there is not that much gained from having a gain drift much better than reference voltage drift.

[Alex Nikitin]

For the more normal +-xx V range I would prefer an ADC with bipolar input, like the LTC2410 or ADS1234. The ADC1210 would be even available in a package suitable for easy hand soldering (SO18)  - though kind of expensive.

ADS1282 looks interesting for this kind of application (not cheap though).

Cheers

Alex

[Andreas]

  For the more normal +-xx V range I would prefer an ADC with bipolar input, like the LTC2410 or ADS1234.

They do not have bipolar inputs. The term is "differential". That means that you can measure negative differences within 0V and VCC.

The ADC1210 would be even available in a package suitable for easy hand soldering (SO18)  - though kind of expensive.

what do you want to do with a 12Bit converter?

Some of the Ti converters work up to +-5 V and thus a useful +-4 V range with good INL.

But I want to measure 10V at least. So 4 V is not a good option even with a precision 2:1 divider.

For a DMM there is not that much gained from having a gain drift much better than reference voltage drift.

I compensate the reference voltage drift to less than 1ppm/30 K = 0.03 ppm/K.
So a ADC gain drift of 0.02 ppm/K is not "much better".

Do you have a better modern ADC which has those features of the LTC2400?

e.g the LTC2508-32

could be worth a try with some tweaking like 50 Hz supression. The device seems to be optimized for 60 Hz line frequency.
I hope that the INL is more like in the first picture (under which conditions?)
But how do I solder such a device?

with best regards

Andreas

[branadic]

But how do I solder such a device?

Not a real problem. I solder such things with an iron with 0.2mm tip by hand, but you can also use a stencil and a hot air rework station. If you need help, let me know. If you have a stencil we could also use one of our reflow soldering ovens.

-branadic-

[try]

many years ago I was interested to build a voltmeter from c't magazine (c't 02/2008, page 170).
It was part of a DIY project called "c't-lab".

Unfortunately it is in German but with schematics, parts list and details about design decissions.
Maybe this could be a helpfull starting point.

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

Hello,

a good starting point: yes. (it is a 5.5 digit design).
I got also some inspiration of the design (especially the R/C filter directly at the ADC).
But there are some mis-conceptions.

Input voltage divider built from standard 0.1% metal film resistors instead of a much better ready made divider.

This is true, but easy to fix. Remove the resistors that compose the discreet decade divider by an integrated one, like Vishay CNS471A5 or the equivalent made by Caddock (Caddock divider recommended to me by Edwin Pettis).

2.5V band gap reference and +/- 2.5V measurement range, which increase linearity, but increase the relative noise by a factor 4.

This is easy to overcome as well. Replace the LT1019 by a MAX6325 for instance.
There had been a design choice in favor of 2.5V. One reason was to avoid people being able to apply 1000V to the device.
With the base measurement range down to 2.5V the maximum allowed voltage is 250V.
Unfortunately that range choice requires operating the divider when measuring references of 7V oder 10V.
The linearity improvement applies for positive Volt figures only. The explanation follows down below.

A LM399 reference is option but uses a trim-pot and metal film resistors which spoil the good T.C.

You can replace the the trim-pot by a two resistors. The trim-pot is not required for an exact adjustment.
You can easily have your base range end slightly above 2.5V.
The exact adjustement ist done in software.

There are other more important issues.
When using a LM399 the design follows the 10V buffered reference as outlined at the beginning of the LT spec sheet.
The current supply for the LM399 relies on the stability of the standard 15V regulator.

One way to circumvent that issue would be to plug a self-referenced 2.5V source into the DIP socket which holds the MAX6325 in my case which can easily be constructed based upon the "portable calibrator" scheme.
I haven't tried that one out yet I have to admit.

The remaining issue is to include an auto zero function somehow.

The multimeter project within the c't-lab series extends the 2.5V range using an offset and a division by two to extend the measurement range from -2.5V to 2.5V. The side effect is that you eliminate simultaneously a big part of the predictable ADC linearity error when adjusting the zero point and the end of scale.

Follow the link and search for the graphic labeled "ADC.png".
https://www.mikrocontroller.net/topic/376240
The blue line shows the typical INL error in ppm.
The x-axis show the values of a fictitous 8 bit ADC, replace that by the 24 bits of the LTC2400.

By calibrating/adjusting zero and positive scale-end you arrive at the pink line.
Most of the INL error to the right of the 0V-point is gone, but it comes at a price when measuring negative volt figures to the left of the 0V-point.

Regards
try

[Andreas]

Replace the LT1019 by a MAX6325 for instance.

I have large ageing drifts with 2 MAX6350 (5V version of the MAX6325).
So probably a LT1019 in metal can is even better than a MAX6325.

with best regards

Andreas

[HalFET]

I've been working on my own 6.5 digit design for months, it ain't a trivial task! Especially if you're trying to design down to a cost like I am.

A few observations of mine:

• An off-the-shelf ADC doesn't do the job, unless you either sacrifice sample rate or your noise floor. I tried three from LT/AD, and another one from TI that seemed to be ok according to datasheet, but in the end they all messed up once you started looking at the temperature sensitivity and repeatability over time. I compared them to the values I got from a calibrated Keithley 2001 at the same time, the reference used was a weston cell I might add, the Keithley 2001 was measuring at the input of the ADC in each case. My conclusion was simple: don't try to push an easily available ADC beyond 5.5 digits, it typically will run into noise or stability issues and will only reach the theoretical 6.5 or 7.5 digits under very specific conditions which are near impossible to achieve in a real world multimeter circuit. I suppose this is the reason why many companies still seem to roll their own for these high resolutions.
• I found the LM399 to be the best choice due to availability, other references tend to have some supply issues at times. Additionally it fits nicely with the voltage range of a lot of the cheaper transformers.
• You're driving the LM399 completely wrong if that's your approach, never put a trimmer like that near a reference of that calibre. You can self-bias it much like you can do with the LTZ1000, the cheapest way to drive them I found is two LM317s in a servo configuration to increase PSSR and a variant on the calibrator circuit in the datasheet. It's best to go for a good quad opamp (e.g. AD704) in this arrangement since it tends to cut cost. You can then also use the remaining AD704s to buffer/fan-out the reference to your ohm's range current source etc.
• You can get away with using regular metal film array resistors to divide the reference to the correct values sometimes, some of them are thermally quite well connected to each other and make for a good enough divider. (I've had a fair amount of luck with components from Welwyn for this.) But you can also get some Caddock ceramic hybrids of Farnell if you desire, if you experience too much noise pick-up on those you can shield them by covering them in copper tape and soldering a wire to the tape and grounding the "shield" that way.
• Implementing the ADC as a discrete circuit using an FPGA seems to be the way to go, The main issue here is that you have to model the charge injection with the switches very well, that seems to be the main caveat for this design, if you need inspiration you can find the service manuals of some of the existing 6.5, 7.5, and 8.5 digit meters.
• The main importance here is that your timing is spot on repeatable, if it isn't you'll never get the timing correct.
• Do use a good ADC to monitor the integration capacitor.
• Never ever use trimmers, fix things in software.
• Go for a temperature stable circuit that ages well, try to achieve things with ratio's and use resistor arrays whenever possible and avoid trusting one "golden unicorn" component.

[Kleinstein]

  For the more normal +-xx V range I would prefer an ADC with bipolar input, like the LTC2410 or ADS1234.

They do not have bipolar inputs. The term is "differential". That means that you can measure negative differences within 0V and VCC.

The ADC1210 would be even available in a package suitable for easy hand soldering (SO18)  - though kind of expensive.

what do you want to do with a 12Bit converter?

Some of the Ti converters work up to +-5 V and thus a useful +-4 V range with good INL.

But I want to measure 10V at least. So 4 V is not a good option even with a precision 2:1 divider.

For a DMM there is not that much gained from having a gain drift much better than reference voltage drift.

I compensate the reference voltage drift to less than 1ppm/30 K = 0.03 ppm/K.
So a ADC gain drift of 0.02 ppm/K is not "much better".
....

If the reference drift is compensated to get a really low drift, one can include the gain drift of the ADC as well. The typical procedure for setting up the compensation will do that anyway.

Of one would not use the 12 bit ADC1210, but maybe a ADS1210.  Sorry for my error.

@HalFET:
For an integrating ADC, one can get away without modeling charge injection, if a PWM like mode is used. The only assumption needed is that charge injection like leakage will stay constant over short times. Unless one wants very high speed, a µC can be an alternative to an FPGA for controlling the integrating ADC. I totally agree that using an ADC to look at the integrator output is a good idea - depending on the mode used the requirements may no be that high - this might even be the cheap µC internal ADC.

[e61_phil]

For an integrating ADC, one can get away without modeling charge injection, if a PWM like mode is used. The only assumption needed is that charge injection like leakage will stay constant over short times. Unless one wants very high speed, a µC can be an alternative to an FPGA for controlling the integrating ADC. I totally agree that using an ADC to look at the integrator output is a good idea - depending on the mode used the requirements may no be that high - this might even be the cheap µC internal ADC.

@HalFET & Kleinstein:
Do you have experience in building such an ADC? Do you think it is possible to build an ultra-linear ADC (like the HP3458As one) with modern components without a special ASIC?
In my opinion the ADC (and its linearity) is still the key feature of the 3458A. Sometimes the speed of the 3458A-ADC is also nice, but even a low speed ultra-linear ADC would be a very nice transfer tool.

[HalFET]

For an integrating ADC, one can get away without modeling charge injection, if a PWM like mode is used. The only assumption needed is that charge injection like leakage will stay constant over short times. Unless one wants very high speed, a µC can be an alternative to an FPGA for controlling the integrating ADC. I totally agree that using an ADC to look at the integrator output is a good idea - depending on the mode used the requirements may no be that high - this might even be the cheap µC internal ADC.

@HalFET & Kleinstein:
Do you have experience in building such an ADC? Do you think it is possible to build an ultra-linear ADC (like the HP3458As one) with modern components without a special ASIC?
In my opinion the ADC (and its linearity) is still the key feature of the 3458A. Sometimes the speed of the 3458A-ADC is also nice, but even a low speed ultra-linear ADC would be a very nice transfer tool.

There's a reason I'm aiming for 6.5 I've noticed non-linearities in opamps already at this point. You really want to keep those supply rails far away from your signal and use a double sided supply,.

And yeah it is indeed theoretically possible to eliminate the charge injection by doing the switching right. I've just found it difficult to achieve in practice. I really need to sit down one day and redesign the sampling FSM. And personally I went for the FPGA because I want speed, additionally it was easier to sync the sampling clock with the grid that way for me, though I think I'll let that part of the circuit drop in the next itteration