Designing a 0-10V precision voltage source - Page 1
Forum section: electronics / projects

author: david77
author_rank: Frequent Contributor
timestamp: March 14, 2021, 01:51:58 am
content: Hi guys.I'd like to design my own 0-10V precision voltage source for reference and calibration purposes as well as a general tool for use in the lab.Some specs I had in mind:- 0-10,0000V digitally setable- resolution of 100µV should be possible (maybe down to 10µV, don't know if that's feasible)- low drift, low TC- maybe a few 10s of mA current capability- I'd like to use parts I have lying around as much as possible.- Haven't decided on power yet. Batteries + mains or just mains powered- Do I need to read back the set voltage? Dunno. If so I'd need the same precision on th ADC as on the DAC, surely?I know that's not an original idea and has been done a few times but I view it as a personal challenge to design & build a thing like that.The first decision is what reference to use. I'm aware there are a few very nice ones on the market today but I do have some relatively nice ones here already.What I've got so far are a few AD584KH, LM3999Z and REF02C voltage references and some LM399 are on their way from Bulgaria.I've also built a small oven to keep the reference at a stable temperature.The oven uses four resistors attached to Hammond 1590B die cast enclosure as heaters and is controlled by the simple PWM controller shown in this TI app notehttps://www.ti.com/lit/an/snoa353b/snoa353b.pdf?ts=1615684455612&ref_url=https%253A%252F%252Fwww.google.com%252F.The oven is under test at the moment and it looks like it's keeping temperature within 0,1°C. I think that should be fine.I also made a first test with the AD584KH inside, that stabilises at around 9,99995V with temp set to 32,9°C. Sadly I don't have access to any suitable data loggers.What I'm thinking now is I'll use the AD584 and add an op as a buffer inside the box to generate a 10V reference for a DAC that I'll drive with some Atmel processor to set the output voltage. I reckon I'll ned a 16Bit DAC at the very least, probably 18 or 20 bit would be better.Somebody on here made a similar device using the AD5791 which seems a good, if expensive, choice.The AD584KH seems reasonably good to me, any thoughts? Yeah, I'd love to use an LTZ1000 but wont. Price is the issue here.I'm very tempted to simply use a 10V reference. Should I? The AD584 can be set to 2,5 or 5V as well.I have a preference for refs with tight initial tolerance band, as I do not have access to a recently cal'ed meter with a high enough resolution. I own a Fluke 8502 and have a Rigol DM3068 at work.My experience using DACs is severly limited. First question is how much of an issue is the references noise? How much will I be worrying about INL & DNL with a high end DAC like the 5791 or 5781? From a previous project I remember it was quite a struggle. Is it worth getting such an expensive DAC anyway? Any suggestions on another DAC? Would it make any sense to put the DAC inside the oven?I've read through a lot of topics on voltage refs here on the forum but I'm sure there will be more questions cropping up.This is not supposed to be any sort of cal standard, but even so I'd like it to perform reasonably well for advanced hobby use.I'd appreciate your input & criticisms.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: March 14, 2021, 04:15:41 pm
content: The difficult part here is not the voltage reference, it's the DAC. You have this which match all your requierements, minus the 0-10V output :https://www.analog.com/media/en/technical-documentation/tech-articles/lt-journal-article/LTC2400_0501_Mag.pdfOr the newer AD5791 that's fully integrated.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2021, 05:00:53 pm
content: The AD584 is a relatively noisy reference, not really a good starting point. The Ref02 looks a little better. Depending on the DAC one may need a little more than 10 V to start. Sometimes 2 reference in series may be alternative to amplification.The difficult part is the DAC part and maybe amplification of the reference if needed. There are also lower cost versions like DAC8801, more adequate for the AD584. A highly stable DAC is expensive however. It is a balance of price and performance. The advantage of a good DAC chip is that it is fast and could also produce more than just DC levels.The alternative to a DAC chip would be a PWM type DAC circuit - still quite some circuit and one would likely need some way to test the linearity. Some designs need an auxiary ADC for some internal adjustments ( coarse and fine part).Another possibility is lower stability DAC (could be 2 combined) and than a much better ADC to check the actual output voltage and correct as needed. However most highly stable ADC chips are more like 2.5 to 5 V input only. So one would still need a stable 2:1 or similar divider. In this case the reference to start with would be some 5 V for the ADC - so not well suited for a LM399 or LM3999.Depending on the construction one may not need to read back the voltage. Reading back and correct is just one possible way.

author: dietert1
author_rank: Super Contributor
timestamp: March 14, 2021, 06:52:17 pm
content: The LM399 thread in the metrology section contains a lengthy discussion of how to make a PWM DAC. At least they tried to propose a circuit and characterize it. Starting from there i was able to make a PWM DAC for the 7 -> 10 V gain stage of a LM399 reference, with good precision (about 2 uV). The circuit is a bit complex:- run filter caps at near 0 V by bootstrapping them with large electrolytic caps.- separate supply and signal grounds- re-synchronize PWM signal out of MSP430 port for lowest jitter- use sigma-delta modulation to get high resolution- use ripple compensation to reduce AC load at filter output.It still needs work. Ripple compensation will help to reduce PWM clock from 10 KHz to 1 KHz. And i want to use a discrete switch as single bit DAC, since it will have much lower delay and temperature dependent delay. And the switch resistance can be less than 1 Ohm. There is a prototype with a dual n-channel mosfet.Making this kind of precision PWM is a project of several weeks.Regards, Dieter

author: Terry Bites
author_rank: Super Contributor
timestamp: March 15, 2021, 11:02:21 am
content: Have you seen this?https://www.analog.com/en/analog-dialogue/articles/high-precision-voltage-source.html