Can a precision DAC be used as a voltage reference?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 05:09:24 am
content: Title.Can a precision DAC (say 16bit or more) be used as a voltage reference? The DAC does have a precision voltage reference inside, which I assume would allow it to produce precision voltages?

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2023, 05:17:21 am
content: Define "precision" in terms of numbers and then compare the parameters for the reference and inaccuracies introduced by the DAC. If the resulting accuracy is still good for your needs, then sure, why not?You may need to buffer it, since unbuffered DAC output typically has limited maximum load.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 05:20:35 am
content: Quote from: ataradov on September 19, 2023, 05:17:21 amHonestly, I'm not sure what I mean by "precision". I have no precision measuring equipment but I'd love to get some precision voltage references for the cheap. I was just thinking it may be possible to make one with a simple DAC?In terms of buffering the DAC output, could I simply use an op-amp in unity gain or would that mess up the output voltage of the DAC?

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2023, 05:23:21 am
content: There is no abstract "precision". You need to know what you want from the reference. Any voltage is precise enough for some use.Opamp buffer is fine. Again, you would have to factor any inaccuracies introduced by the opamp into the accuracy of the final value.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 05:26:23 am
content: Quote from: ataradov on September 19, 2023, 05:23:21 amhttps://www.eevblog.com/forum/testgear/40-000-count-multimeter-under-$50-holdpeak-hp-770d/Essentially I was thinking that I could calibrate the HP770D multimeter myself, using a DAC to provide a precise voltage. Would using a DAC be a good way to calibrate the DC voltages of the multimeter?

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2023, 05:32:25 am
content: How exactly are you going to calibrate it?It is up to you what you consider accurate enough reference. To the math based on the specific DAC you want to use and if the resulting accuracy is good enough, then you can use it to verify the meter.There is also no need to buffer it for this use.But generally, you will still be verifying against an unknown value. The whole point of standard references is that they were calibrated against a more accurate source.

author: Berni
author_rank: Super Contributor
timestamp: September 19, 2023, 05:33:43 am
content: Yep as said above it all depends on how exactly you define precision for your use case.Will it be accurate within 1ppm? No way in hell.Will it be accurate within the last digit of a 2.5 count DMM? Yep, easy, any decent DAC should be within that precision.In terms of home lab meteorology, having a adjustable reference is not that terribly useful. Having a rock solid fixed voltage reference is better than having a adjustable okayish reference. Without going into any volt nuttery the way to go about precision is to just buy a decent brand name 6.5 digit bench DMM and just trust it. Helps to get it checked against something like a known reference of someone elses precision DMM. There is not much practical use for excessive precision in a home lab.Once you have a 6.5 digit DMM you can use that to test the precision of everything else in your lab, including things like characterizing the tempco and drift of any references you make.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 05:37:29 am
content: Quote from: ataradov on September 19, 2023, 05:32:25 amI would have no way to calibrate it. I guess it's wrong to assume that a high precision DAC would just naturally already be precise (ex. being told to output 1v, and it outputting near-exactly 1.000v).Alright so I guess it really doesn't make sense to try to use a DAC to calibrate my multimeter, without first calibrating the DAC itself?

author: ataradov
author_rank: Super Contributor
timestamp: September 19, 2023, 05:40:53 am
content: Quote from: LooseJunkHater on September 19, 2023, 05:37:29 amIt is not wrong, but the exact value of "near-exactly" would depend on a lot of factors.Quote from: LooseJunkHater on September 19, 2023, 05:37:29 amEven if the DAC itself is the best thing out there, you will be using unverified design. May be you will have some flux residue on the board you will build, which is slightly conductive. How would you know without verifying your design against some known standard?

author: Berni
author_rank: Super Contributor
timestamp: September 19, 2023, 05:55:35 am
content: Multimeters don't really need calibration at multiple points, so the adjustable aspect of it is not that useful.Generally multimeters are very linear by design. so as long as it is a decent DMM you can just apply a fixed known voltage and check that voltage is correct. You can also check for negative by swapping the probes and for 0 by shorting them. Then you can range the DMM up 1 or 2 ranges to get a few other points and make sure the dividers inside the DMM for those ranges are good.The more tricky part is small voltage ranges, but you can divide that down, check it with a higher range, then range down and see it still fits.If you want i can measure a reference for you on my 8.5 digit Keithey for the price of postage of sending it back to you afterwards.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2023, 06:44:44 am
content: The multimeter should be a relatively new type and there is thus normally no need to do a calibration / adjustment. With such handhald meters It would be mainly a check for plausibilty and if for some reason the meter is way of it is broken with usually no way to really repair it. A re-adjustment would not fix a problem and further dirft is likely.The recent video of Dave on the repair of a HP OLED meter is an exception - he found the reason for the drifting readings and fixed the problem (good cleaning with desoldering the critical part).

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2023, 06:54:00 am
content: To calibrate or adjust your 40000 meter with LM385-1.2V Vreference inside you would need an external equipment with say, 10x better parameters. Also the question is how stable are the internal dividers and the ADC itself (and how is their thermal coefficient) in that 770 meter.The LM385Z-1.2 is up to 150ppm/C (the worst spec, X/Y are 30-50ppm/C, Z means TO-92 package) and 20ppm/1kHr (TC based on suffix, see DS) thus you may compare those params with your DAC's params.PS: I would estimate the reference around 50-70ppm/C (I have one with 60), not considering the long time drift. Now with say 60ppm/C and a 5 degrees Celsius difference in your lab the 4.0000V will show you 3.9988V or 4.0012V based on TC sign..

author: EPAIII
author_rank: Super Contributor
timestamp: September 19, 2023, 10:27:28 am
content: You say you want to "calibrate" your meter. But do you know the meaning of that word in professional electronics?So I ask, do you want to go through a complete calibration, following HP's procedure for this meter and adjusting all internal adjustments called for in that procedure, using HP's recommended methods and materials? That is what "calibration" really means.OR, do you just want to find out if the meter is in specification at ONE DC Voltage and therefore likely to be OK for others. And also for DC current, AC Voltage and current, resistance, and whatever else. This is a confidence check, not a calibration. And the further you get away from that ONE DC Voltage range, the less likely it will be that you are ensuring anything.If you want a real calibration, the first thing you must do is get the HP calibration procedure. Otherwise you are just stumbling around in the dark. Good luck!. Then, your Voltage standard is going to need to be at least 10 times more accurate than your meter. Your meter has a basic, DC spec. of 0.1% or one part in a thousand. So you need a Voltage standard that is accurate to 0.01% or one part in ten thousand. And then you also need whatever other standards the HP procedure calls for.If all you want is a confidence check, then that can be accomplished with a lot less. There are inexpensive Voltage reference devices sold on the internet with a 0.3% accuracy or perhaps slightly better. YOUR METER has more accuracy than that so you should NOT attempt to change any of it's internal settings when using such a device. But they can give you some amount of confidence that it is working properly on one or more scales. This is just one such device; there are many.https://www.amazon.com/Acxico-Precision-Voltage-Reference-4-Channel/dp/B0B4WN2Q91/ref=d_m_crc_dp_lf_d_t1_sccl_4_2/146-5667566-5396205?pd_rd_w=5aCIf&content-id=amzn1.sym.5d471845-5073-424b-b27b-c0676f48a016&pf_rd_p=5d471845-5073-424b-b27b-c0676f48a016&pf_rd_r=TRP490ZM4R6RCVTHTC91&pd_rd_wg=Telwd&pd_rd_r=fb4ee8bd-a286-43d7-9462-3af1ea49cbf4&pd_rd_i=B0B4WN2Q91&psc=1For resistance you can purchase 0.1% or better resistors for a reasonable price. One or two such resistors per resistance scale would also provide a confidence check there.AC Voltage may be somewhat more problematic. I do not know of any inexpensive AC Voltage sources. Perhaps others here can help there.DC current, at least low values of DC current can be checked with a Voltage source and series resistor. But AC current, once again would be more difficult.Once again, I DEFINITELY would NOT make any internal adjustments to your meter using any of these less accurate, make-shift devices. Your meter is probably more accurate than they are.I will say one more thing about meters and some here will probably disagree. Those people probably come from industries where high precision is absolutely necessary or where even a lower level of precision must be accompanied with regular calibrations to ensure it. I don't argue with them. They know where they come from. But I have worked in electronics for over 45 years professionally and with electronics for at least another 15 years and I have almost NEVER seen a meter that I worked with just drift out of it's basic accuracy. They either worked IN SPECIFICATION or they just DID NOT WORK at all. That includes analog and digital meters up to at least the 0.1% accuracy that yours claims. It does not include super accurate and expensive, lab style meters with accuracy out to 6, 7, 8, or more digits. Yes, they WILL drift and calibration is a necessity. But it most certainly does include the $13 meter you are talking about.

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2023, 10:38:52 am
content: Btw. the "calibration" means to compare the meter's readings against a specification for given meter, the output of the procedure is a written report saying for example your 4V range is 0.036% off while reading a 4.00000V external reference, what might be (or might be not) in the meter's spec (an example only).The "adjustment" means your meter will be set (in sw or hw or both) such it reads the required level, like your 4V range now reads the 4.00000V external reference as "4.0000V". The output of the procedure is the adjusted meter..Thus calibration and adjustment is not the same (in $$ as well).

author: Infraviolet
author_rank: Super Contributor
timestamp: September 19, 2023, 02:37:08 pm
content: My guess is that what LooseJunkHater is trying to work out is whether the voltage from a DAC will stay stable even if the supply voltage to the DAC varies, versus whether the DAC's output will be more like X% of whatever the supply is providing?

author: TimFox
author_rank: Super Contributor
timestamp: September 19, 2023, 03:30:52 pm
content: Another example of excess handwaving.The DACs voltage reference should be more stable than the supply voltage to the DAC, but everything past that is aquantitativequestion.To check calibration of another object, you need to know the manufacturer's specification on the accuracy of that voltage reference: not all DACs require tight accuracy on the reference.

author: tszaboo
author_rank: Super Contributor
timestamp: September 19, 2023, 03:55:11 pm
content: Yes, I've seen quite a few DACs from TI and AD thar had a high quality, well defined VREF inside. If you want to use it for something else buffering migh be needed. And you need to be aware of the specifications you need.

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2023, 04:28:45 pm
content: The answer on the OP's question is yes, of course a "precision DAC" could be used as a voltage reference. The average DACs today have got +/-1ppm INL, +/-1ppm DNL, 1-2ppm/C gain TC that is by order of magnitude(s) better than his PeakHold 770d would require.What is needed is the voltage reference for the DAC, and usually a buffer for the output.Also for calibration of such a multimeter you may need different voltages, like 400mV, 4V, 40V, 400V and that makes the stuff much more complicated.For the 4V and 400mV almost any DAC with a 3-5ppm/C reference with some reasonable long term stability, preferably in metal package would do. You may need one or two opamps (zero drift one) and a couple of 5ppm/C resistors to make the stuff working. With a small MCU feeding the DAC with data you may even compensate for temperature and linearity, add a small display and you get a nice DC calibrator.I bet the cost of components for such a calibrator could be lower than the costs for calibration of a single range of his multimeter in a professional lab in DE..

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 07:12:03 pm
content: Quote from: iMo on September 19, 2023, 06:54:00 amWow, I did not expect 5C to actually lead to a difference of +- 2.4mv (which I guess isn't much?). Do more expensive meters like the Flukes and Keysights also suffer with such problems, or do they have internal compensation?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 07:18:34 pm
content: Quote from: EPAIII on September 19, 2023, 10:27:28 amThis is more like what I want. I'd love if my meter could accurately read voltages from 10mv all the way to 10v; outside of that range, I really don't care much. If I can't "calibrate" my meter to read those ranges, it would be good for me to note-down how far-off my multimeter is measuring such voltages (so that I could externally account for it).I am aware of those AD584 modules, but they all appear to use fake AD584 references and I don't know if I should trust their voltages?

author: BeBuLamar
author_rank: Super Contributor
timestamp: September 19, 2023, 07:25:37 pm
content: I don't think a precision DAC is more precise than my DMM.

author: radiolistener
author_rank: Super Contributor
timestamp: September 19, 2023, 10:09:27 pm
content: Quote from: LooseJunkHater on September 19, 2023, 05:09:24 amDAC output depends on it's reference input. So the answer depends if reverence voltage connected to DAC reference input and DAC non-linearity and noise is acceptable for your needs.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 19, 2023, 11:29:13 pm
content: In the past several methods have been used to make an adustable voltage from a fixed voltage reference.The Kelvin-Varley divider is probably one of the oldest:https://en.wikipedia.org/wiki/Kelvin%E2%80%93Varley_dividerIt can be better then 1ppm linearity, but you pay for hand sorting and matching resistors, that also need to be stable over time and temperature. (Some had the resistors in an oil bath to keep their temperature stable).Later came the PWM based standards. PWM has an inherent linearity and with a bit of careful design it can also be better than an 1ppm linearity, but they avoid a lot of the inherent cost of the Kelvin-Varley. I'm guessing that this method was popular in the '80-ies and '90-ies. (And it may still be used).Over time IC's have become better, and there are DAC's that reach into the 1ppm linearity domain and are good enough for a lot of voltage reference purposes. See for example the link below.https://www.analog.com/en/analog-dialogue/articles/20-bit-dac-and-accurate-precision-voltage-source.htmlBut for regular handheld multimeters you do not need equipment like this. Those are usually calibrated with a fixed voltage.