AD5791EVAL INL changes with different references - Page 1
Forum section: electronics / general

author: splin
author_rank: Frequent Contributor
timestamp: February 21, 2020, 06:34:22 pm
content: It was pointed out recently on the LTZ1000 thread that ADI sell an LTZ1000A board. Interestingly the EVAL board documentation shows very different INL plots for the AD5791 20-bit DAC when using 3 different voltage reference daughter boards.I find this surprising as:a) The AD5791 Vref+ and Vref- inputs on the main board are buffered with AD8676 opamps.b) All three vref daughter borads buffer the vref+ and vref- ouputs with AD4077 opamps. The main difference between them is that the o/p buffers also translate the reference voltage to +/- 10V from 7.2V in the case of the LTZ1000, and 5V for the others.Can anyone suggest why there is such a big difference between the LTC6655 and ADR445 INL plots in particular, given that the only real circuit difference is the reference itself and that there are two, identical, opamp buffer stages between the reference and the DAC in both cases? The ADR445 has higher ouput impedance, +/-50ppm/mA (max) compared to the LTC6655BMS8's +20, -30ppm/mA (max) but surely the two subsequent buffer stages must make the loads on the references insignificant.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2020, 07:33:10 pm
content: The difference is in deed odd, especially as the reference circuits for the ADR445 and LTC6655 are relatively similar.I could imagine that there is some thermal interaction from the DAC effecting the reference part. So one may see an effect of individual TC. Other units may behave different.Another possible point could be a power supply issue with the ADR445.As the curves look also different in the details, this could be different board used in the tests and also just a different temperature can have an effect. Ideally one would run such a test at least twice to also check the repeatability. It does not need that much temperature drift during the run with the ADR445 to cause 1 ppm of error.

author: jbb
author_rank: Super Contributor
timestamp: February 21, 2020, 07:42:50 pm
content: Wild-arsed-guess here...That DAC's got a lot of bits, which would make INL tests difficult. Could the different noise profiles of the different references be sort of 'aliased' into INL with their test scheme?

author: Echo88
author_rank: Frequent Contributor
timestamp: February 22, 2020, 08:31:07 pm
content: INL-Test takes some time so we see the short-term-stability (and maybe also TC) of the references? ADR shows worst and LTZ best INL-performance, so assumption might be justified.

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2020, 08:55:11 pm
content: Hello,With my LTC2400 based ADCs I also have relative large stray from device to device even with the same type of reference.ADC13 shows 13.2 ppm INLADC25 shows 6.7 ppm INLBoth are with LTC2400 and AD586So I think the "differences measured with AD5791" is normal stray from device to device. (all well within the +/- 1(.5) ppm spec of the AD5791)For the AD5791 there may be some dependancy on output impedance on the reference (since the VREF impedance is code dependant according to the data sheet). But since the voltage reference is not directly connected to the reference this is not the reason here.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2020, 09:34:37 pm
content: If the curves are just measured once, it absolutely makes sense to explain them with instability in the reference. 0.2 ppm for the LTZ1000 is still only 1.4 µV and thus still within the range of instability over some 350 seconds it takes to get 1024 voltage readings.

author: splin
author_rank: Frequent Contributor
timestamp: February 23, 2020, 06:36:46 pm
content: Quote from: Andreas on February 22, 2020, 08:55:11 pmI rather doubt that - the spec is .25LSB typ, +/- 1ppm max 0C > T < 105C, and the ADR445 graph shows almost 2ppm INL which is out of spec. That's odd in itself as manufacturers are usually very careful to show the performance of their devices in the best possible light, often by testing in conditions which are not realistically acheivable in real applications. So to publish a graph showing the part to be out of spec must surely be a mistake - possibly reflecting declining engineering standards in companies under a great deal of pressure from competition, and especially in the precision analog part of the market which is becoming ever more niche.QuoteAgreed. Sadly, I think Kleinstein is probably right:Quote from: Kleinstein on February 22, 2020, 09:34:37 pmI say sadly because it reflects badly on standards at ADI - either the engineer who ran the test didn't check the results were valid and (repeat them when it turned out that they weren't due to poor test setup or an out of spec AD5691) or worse was forced to publish them anyway due to time/resource pressures.I would have thought it likely that the three tests with different voltage references would have used the same AD5791 evaluation board and plugging in the appropriate reference daughter board. In which case, as suggested by Kleinstein, the ADR445 INL plot probably reflects instability in the reference over the prolonged test period. The 3458A (used for the INL measurements) achieves .01ppm rms, .06ppm p-p noise at 100NPLC which would be the minimum required to test sub 1ppm INL. I don't know how many points are measured in their INL tests but even 1024 would take 47 minutes, or double with autozero. It wouldn't surprise me if the ADR445 stability is not good enough for this test over those durations.The tests should have been done using the 3458A ratio measurements to eliminate reference drift, but that would have taken even longer and introduced more noise whch have required compensation by even longer apertures (NPLCs).

author: splin
author_rank: Frequent Contributor
timestamp: February 23, 2020, 07:05:29 pm
content: It seems AD have finally got some competition for the AD5791 - TI's new 20 bit DAC1101A. It's doesn't quite match the AD5791 for INL, +/- 2.6ppm max v +/- 1ppm, but other specs are better:0.1 to 10Hz noise 0.4uV p-p V 1.1uVTHD @ 1kHz -105dB V -97dB (400kHz update rate V 10kHz)Mid code glitch 1nV-s V 3.1nV-sZero/midscale, fullscale and gain temperature drifts are very similar.Biggest factor is that the AD5791B now costs over $100 (1 off) but the TI part costs $52 (or $68 v $33 direct from AD/TI 1k off). I'd expect two of the latter in parallel would be very close INL wise to the AD5791 but be very much quieter which would be a significant benefit in many aspplications, especially test setups.

author: jaromir
author_rank: Unknown
timestamp: February 23, 2020, 07:43:27 pm
content: Quote from: splin on February 23, 2020, 07:05:29 pmDAC11001A is indeed interesting part, one of a few precision DACs that allow Vref higher than AVdd (typically 5V maximum), enabling to use LM399 or LTZ1000 based references with no down-scaling.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2020, 08:26:20 pm
content: The 0.1 -10 Hz noise specs for the TI part is with a +10 and 0 Reference, while AD uses +-10 V. Expect the 1/f noise of the Ti part to be nearly twice as high with a +-10 reference. Still slightly better, but not much any more.The AD5791 also supports a higher analog supply. So the chips are quite similar, not directly exchangeable, but close.

author: MiDi
author_rank: Frequent Contributor
timestamp: February 26, 2020, 06:45:49 am
content: Asking me what is the source of voltage dependent (1/f) noise?I would not expect the resistors to have significant voltage dependent noise.Do the switches for the R2R ladder exhibit voltage dependent noise?What I would expect is code dependent noise contribution.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2020, 07:37:52 am
content: Thin film resistors tend to have some 1/f noise proportional to the voltage applied. Chances are this is the main source of 1/f noise in the ADC.Besides the resistors in the ADC itself, the generation of the negative reference (especially if using chip internal resistors) can also contribute.