AD7172-2 Noise tests. - Page 1
Forum section: electronics / general

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 02:13:59 pm
content: Subject of my research is oulined here:https://www.analog.com/en/analog-dialogue/articles/why-does-voltage-reference-noise-matter.htmlShort summary:1. Noise measurements of the ADC with input pins shorted - absurdity.2. Simple LPF using ultra low leak polarized electrolytic capacitors is very efficient way to filter internal /external V-ref.Tests results.Input always 2V, filtered low noise source obtained with help 10k & 1.500 uF RC network. Internal adc buffers active for both,IN & Ref-IN.1. V-ref selected internal - 2.5V, using adc register 0x20 setiings. RMS value: 10-11 (LSB)2. V-ref selected external - 2.5V, switch on, low time constant. RMS value: 5-6 (LSB)3. V-ref selected external - 2.5V, switch off, big time constant. RMS value: 1-2 (LSB)For reference, noise with input pins shorted ~1 LSB.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2023, 03:21:13 pm
content: The noise with a shorted input still makes sense, as one point for the noise testing. This however only applies with a short before the input buffers, not the ADC in unbuffered mode and than an short.Reference side noise may well be more important with a voltage near the full scale. Still quite some measurements would be done with way less than full scale.The filters used for the reference and input look like they could effect the accuracy. One point here is dirft in the input current and the other point is input current to the filter capacitors. The capacitors react to temperature and mechanical stress and because of the DA settling can take a long time (like a few hours). The filter helps with the nois, but will reduce the accuracy.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 04:27:47 pm
content: Ref. Input curent with buffers on = 100 nA. Since there is constant voltage & sampling clock frequency - no issue is expected with linearity.Voltage drop accross 10k resistor = 1 mVInitial accuracy of the internal voltage reference +-0.12% or +-3mVAs you can see accuracy is not sucrificied.Capacitor leakage current is ~10 nA or 1/10 of the ref. inputs, the only concern its may rise with temp., than active LPF would be necessary.Caps tested so far:UKL0J102MPDRNL1C152MDS1

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2023, 04:56:06 pm
content: I see 3 problematic points with the 10 K resistors:The input current of the ref. buffers has quite some drift: +-0.25 nA/K typical from the datasheet. With 10 K and a 2,5 V ref. this tanslates to 1 ppm/K of drift. If the buffer is BJT based, there will also be 1/f current noise that can become a problem. The linked article on reference noise showed an low pass filter using the AD797. It works good for the higher frequencies, but at the lower frequencies it gets worse than it was before.The temperature drift of the capacitor leakage current. 10 nA is quite low, but chances are it is also quite temperature dependent. If it is similar to semiconductor leakace the drift may reach 1 nA/K.The final problem is the DA if the capacitor. With electrolytic capacitors this can real a few percent and is thus needs quite along time to settle after turn on. To get settling to the ppm level it would need some 1000-10000 RC time constants and with the low cross over frequency this gets really long, possibly more than 1 day. Here it is not so much the resistor itself that is the problem, the point of having a long time constant with an electrolytic capacitor.It is a bit hard to tell appart the settling of the capacitors leakage current and DA. For the capacitor leakage (maybe DA) it is no do uncommon to take days for settling.Filtering the low frequency reference noise is a tricky thing. Even film capacitors reactor to thermal disturband and stress / seismic events. A some point the effort for filtering gets larger than simply using a lower noise or 2nd reference chip.This is anyway a topic seprate from the ADC itself, but more thing for the reference. For testing the ADC it should be good einough to have the same reference for the ADC and test signal. In this case there is not need the low frequency filtering - just the higher frequency filtering can be relevent.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:15:55 pm
content: Quote from: Kleinstein on May 31, 2023, 04:56:06 pmIt's all about ADC noise tests with respect DC input voltage measurements, when in general case input signal is not correlated to reference voltage in any way. So situation is just opposite, low frequency noise is matter, high frequency has no meaning for SD ADC at all.Indeed settling time of the filter is quite long, so I came up with simple circuits to disconnect capacitor during power outage..Recorded voltage lost is 60 mV when circuits was w/o power for ~10 hours.

author: iMo
author_rank: Super Contributor
timestamp: May 31, 2023, 06:58:53 pm
content: Did you try to run it in the differential input mode?Like the input 2.5V source floating and the input AIN1 biased to REF+ (for example)?

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 07:16:20 pm
content: Quote from: iMo on May 31, 2023, 06:58:53 pmADC is running in diff. mode, max input range +-2.5V. The purpose of the voltage divider lowering input voltage down to 2V is to avoid saturation.I tested 2 cases, divider driven by Ref voltage taken right from pin 4, and driven by anothe max6164-4.096V divided /2. Of cource, using max6164 shows higher noise level since non-filtered sub 10 mHz range taking power, but not much, instead of 1-2 LSB reads 2-4.In essense, if ADC is used "as it is" w/o reference filtering noise is ~10 LSB, see post #1

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2023, 07:30:09 pm
content: Ideally an ADC would not react to high frequency reference noise. However the way the SD ADCs work is that they sample the reference at a certain frequence / times and from this it is normal that a SD ADC reactors also to higher frequency (e.g. MHz range) reference noice. So some filtering on the reference side make absolutely sense. It is quite normal to have filtering, alone to handle the current spikes that most switched capacitor SD ADCs produce at the inputs. So quite some capacitance at the inputs is normal, though usually with less resistance.For DC measurements filtering is usually not that practical as the times for the measurements are often quite long. For the input signal the settling makes really low frequency analog filtering not practical. Already suppressing the 50/60 Hz mains hum is a bit of a challange and slow settling.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 07:38:23 pm
content: Quote from: Kleinstein on May 31, 2023, 07:30:09 pmYou 've missed the point again, settling time is for reference voltage only in question, and with cap always charged - no issue.Input filtering is not required, I used filter just to create noise-free source, or I can use 1.5V battery otherways.More details on the test set-up:sampling rate - 10 sps, two cahnnales diff. mode, 5 sps per channel. (I used second one to measure/ monitoring drop voltage across 10k resistors)RMS calculated over 128 samples, averaging time frame 24 seconds.LSB ~= 150 nV

author: maat
author_rank: Regular Contributor
timestamp: May 31, 2023, 07:44:29 pm
content: Would you mind uploading the raw data? Or at least plot a histogram? That would make your contribution even more valuable.

author: iMo
author_rank: Super Contributor
timestamp: May 31, 2023, 07:46:15 pm
content: Quote from: MasterT on May 31, 2023, 07:16:20 pmI mean the diff "bipolar mode", where the AIN1 is biased to REF+ for example (the virtual ground).Grounding the AIN1 as you have there works in the unipolar mode only, imho..

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 31, 2023, 07:59:20 pm
content: I'd be nervous about large capacitors anywhere in a reference circuit. As mentioned, the leakage will change with temperature. They will also pump current in and out with every little temperature change. Just seems like a separate research project would be needed to understand the many ways they can influence the circuit.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 09:38:01 pm
content: Quote from: iMo on May 31, 2023, 07:46:15 pmI still don't understand, ADC is "fully differential" - means you free to wire up AIN1 to anything, ground, Vref or Vpower. The only limits is 2.5 V at max. There is no difference in noise level vs common voltage, only linearity may be worse, as they say in DS.

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:04:15 pm
content: Quote from: Conrad Hoffman on May 31, 2023, 07:59:20 pmLet me explain a little bit, seems AD article is too long to read.If I try to build a voltmeter based on AD7172-2, than I would look into table table 21 in DS and expect 0.25 uV RMS , right? Wrong.All those tables were measured with likely inputs shorted, and nobody knows what reference was in use.ADC IS a multiplicator, so Result = V-in x V-ref -> product by two values, and zeroing one (V-in) is dumb stupid things to do.Next, if I apply extremely pure DC at the inputs what digits I should expect to flicker on my voltmeter indicator? 0.25 uV rms means 1.5uV p-p, s0 6-digits at the best. For human end user noise is matter as it looks on display, time term is about 1-10 sec, anything slower - is a drift and doesn't bother me much. If I need to measure a voltage I hope to get settled value in a few seconds, or even less than a sec.I agree that electrolytic is not perfect components, and temperature sensitivity is a weak side, but temperature doesn't change in a seconds - so I underlined ones more - noise/ drift in the 0.1-10 HZ was considered in this research.Why AD7172? Yes, the problem is not appear today, it's always were , but only with extremely low noise ADC (24-bits 1 LSB noise) this problem sharpen itself to the level, that if not taken care off, it's does compromised ADC noise performanse. See post #1, 10 LSB vs 1 LSB.Reference voltage filtering using 1-10 uF cap was sufficient for early SD ADC > 1 uV rms noise, I'm talking about 1uV with G=1 ( disregard 10nV noise ADC with G=128 as a data manipulation).

author: MasterT
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:56:23 pm
content: Quote from: maat on May 31, 2023, 07:44:29 pm1. Internal reference "internaly" mux-ed via reg. 0x20 bits 4:5 = b10AD-7172. Wait, please...*0 6715222 6715205 6715211 6715209 6715224 6715226 6715231 6715228 6715218 6715226 6715224 6715219 6715235 6715214 6715214 671522516 6715216 6715201 6715205 6715210 6715204 6715212 6715210 6715204 6715206 6715217 6715209 6715215 6715222 6715221 6715213 671521232 6715209 6715203 6715203 6715205 6715204 6715213 6715207 6715205 6715207 6715222 6715208 6715222 6715213 6715201 6715203 671521548 6715210 6715217 6715211 6715224 6715209 6715221 6715214 6715212 6715221 6715229 6715225 6715243 6715237 6715228 6715226 671522464 6715222 6715208 6715214 6715214 6715210 6715205 6715218 6715222 6715229 6715233 6715228 6715217 6715229 6715223 6715230 671522680 6715219 6715213 6715223 6715217 6715213 6715218 6715215 6715221 6715212 6715226 6715218 6715231 6715217 6715221 6715225 671522996 6715214 6715219 6715218 6715222 6715235 6715236 6715226 6715225 6715227 6715230 6715240 6715224 6715225 6715235 6715225 6715208112 6715233 6715236 6715237 6715213 6715213 6715210 6715229 6715210 6715235 6715235 6715228 6715210 6715212 6715199 6715201 6715210SNR: 9.909 aver uV: 2001291.0592. LPF in use, switch onAD-7172. Wait, please...*0 6715385 6715385 6715383 6715388 6715386 6715386 6715384 6715383 6715381 6715382 6715379 6715377 6715375 6715380 6715377 671537716 6715378 6715377 6715376 6715374 6715376 6715376 6715378 6715376 6715375 6715374 6715375 6715375 6715374 6715375 6715374 671537732 6715379 6715384 6715385 6715384 6715382 6715385 6715384 6715381 6715380 6715379 6715380 6715381 6715381 6715387 6715385 671538548 6715385 6715385 6715386 6715388 6715388 6715387 6715385 6715385 6715385 6715387 6715387 6715385 6715383 6715383 6715383 671537964 6715378 6715379 6715379 6715377 6715377 6715371 6715370 6715367 6715369 6715369 6715371 6715372 6715372 6715372 6715375 671537580 6715379 6715385 6715389 6715391 6715390 6715388 6715387 6715388 6715389 6715389 6715391 6715389 6715388 6715387 6715385 671538596 6715384 6715382 6715381 6715378 6715377 6715374 6715372 6715374 6715372 6715371 6715374 6715374 6715375 6715373 6715374 6715371112 6715367 6715366 6715367 6715369 6715370 6715370 6715371 6715373 6715372 6715373 6715374 6715374 6715377 6715382 6715384 6715383SNR: 6.252 aver uV: 2001338.9923. LPF in use, switch offAD-7172. Wait, please...*0 6715448 6715446 6715445 6715446 6715447 6715446 6715446 6715445 6715446 6715445 6715445 6715448 6715446 6715447 6715447 671544516 6715448 6715446 6715446 6715446 6715445 6715445 6715446 6715446 6715447 6715447 6715447 6715446 6715446 6715446 6715446 671544832 6715447 6715447 6715446 6715447 6715446 6715447 6715446 6715445 6715448 6715446 6715446 6715447 6715447 6715448 6715447 671544848 6715448 6715449 6715449 6715451 6715447 6715448 6715449 6715448 6715449 6715449 6715447 6715446 6715447 6715447 6715448 671544764 6715447 6715447 6715448 6715448 6715448 6715447 6715447 6715447 6715447 6715444 6715446 6715449 6715447 6715448 6715448 671544980 6715449 6715446 6715449 6715449 6715448 6715448 6715447 6715449 6715448 6715448 6715448 6715448 6715446 6715447 6715447 671544796 6715447 6715445 6715447 6715447 6715446 6715446 6715445 6715445 6715446 6715445 6715446 6715445 6715447 6715446 6715445 6715447112 6715447 6715445 6715444 6715447 6715447 6715446 6715445 6715447 6715446 6715447 6715447 6715446 6715444 6715446 6715445 6715445SNR: 1.272 aver uV: 2001359.0954. LPF in use, switch off, Inputs shorted -);AD-7172. Wait, please...*0 -328 -330 -330 -329 -331 -330 -330 -329 -330 -327 -331 -329 -328 -329 -330 -32916 -330 -330 -330 -330 -330 -331 -329 -331 -330 -330 -330 -330 -331 -331 -332 -32832 -330 -329 -330 -329 -331 -329 -329 -329 -328 -330 -329 -330 -330 -329 -330 -33048 -329 -330 -329 -331 -329 -330 -330 -330 -331 -331 -330 -330 -331 -328 -327 -32964 -329 -328 -329 -329 -329 -330 -330 -328 -329 -328 -330 -329 -330 -329 -329 -32780 -330 -329 -330 -330 -330 -330 -329 -330 -329 -329 -329 -330 -330 -329 -330 -33096 -329 -329 -329 -328 -329 -329 -329 -328 -330 -329 -328 -328 -330 -329 -328 -329112 -330 -330 -330 -329 -330 -328 -329 -330 -330 -330 -330 -331 -330 -329 -331 -330SNR: 0.927 aver uV: -98.196Math function:Code:void snr_ad7172(double &snr_r0, double &mV0, double &snr_r1, double &mV1){double accum0 = 0.0;double accum1 = 0.0;double tempd = INP_BUFF;double tempf = 0.0;for( uint16_t i = 0; i < INP_BUFF; i++ ) {inp = adc_datas;inp = adc_datas;}int32_t tempr;for( uint16_t i = 0; i < INP_BUFF; i++ ) {accum0 += inp;accum1 += inp;}accum0 /= tempd;accum1 /= tempd;mV0 = adc24_mV( accum0 );mV1 = adc24_mV( accum1 );double snr0 = 0.0;double snr1 = 0.0;for( uint16_t i = 0; i < INP_BUFF; i++ ) {tempf = inp - accum0;snr0 += (tempf * tempf);tempf = inp - accum1;snr1 += (tempf * tempf);}snr0 = sqrt(snr0 /tempd);snr1 = sqrt(snr1 /tempd);snr_r0 = snr0;snr_r1 = snr1;}double adc24_mV(double raw){double mVolt = (volts_ref * raw) /8388608.0;return mVolt;}

author: iMo
author_rank: Super Contributor
timestamp: June 01, 2023, 07:07:31 am
content: Quote from: MasterT on May 31, 2023, 09:38:01 pmThe 7172's DS states the input buffer has "true rail to rail" outputs - provided it is true you are free..PS: btw. - while comparing 7177 and 7172 DSes it seems to me the 7172 is the 7177 but with 8 lowest bits stripped off the 32bit data train (plus some minor differences in the sampling frequencies). For example - the clean "24bit" noise free outputs at lowest sampling frequencies (Fig 11 and 14), with a single histogram bar..

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2023, 08:39:45 am
content: It may be at first glance or for a novice use that the noise up to some 1-10 seconds is what matters. However for real use the relevant noise also includes longer times, like a few 100 seconds. This is to includes time scales needed to manually connect / swap cables and similar things.For data-sheets the 0.1-10 Hz range is a defacto standard for the low frequency noise value. However the 0.1 Hz lower frequency limit is more set by the difficulty to get lower frequency analog filtering and not because lower frequency noise does not matter. For a DMM the lower frequencies do matter. It is more that the slightly higher frequencies (e.g. > 1 Hz) are less relevant.Reference filtering is an old topic. The general conclusion is that it is not feasible to filter away the 1/f noise part well below 1 Hz. A fitler can help with the higher frequencies, but if not carefull it makes things worse below the cut off.Instead of high effort with large low leakage capacitors it is usually better to get a better reference. The mentioned max6164 is not a suitable choice for the AD7172 ADC, as it is nearly as noisy as the internal reference and also with quite some long term drift. It would take a better reference, more like LTC6655 or max6325 to make full use of the ADC.For a DMM both the noise with a shorted input and with a DC voltage matters. Having low noise with a shorted input / low voltage still helps as on average on has quite a bit less than full scale voltage. The data-sheets tend to point out the best points, but for the user the worst points may be more relevant, like just a bit over 10% of the range so that one can not yet switch to the next range.One usually can accept some noise from the reference and still use an ADC with lower noise, as quite often the signal source also has some noise on itself. So for highest resolution one would need to average over some time anyway. This only makes sense if the meter is also stable for more than a few seconds. The faster time scale noise can be averaged out digitally (even in the head), but having added drift and very low frequency noise is bad as settling effects from the signal source (or thermal EMF at connectors) can look similar.For the higher resolution DMM it is usually accepted to have RMS noise of about 1 digit and this a little movement there.

author: iMo
author_rank: Super Contributor
timestamp: June 01, 2023, 10:10:03 am
content: FYI - While reading the REF5025 DS (used in my 24bit SAR ADC) I've found similar articles:Quote

author: MasterT
author_rank: Frequent Contributor
timestamp: June 01, 2023, 11:27:07 am
content: Quote from: Kleinstein on June 01, 2023, 08:39:45 am1. Reasobable, to have time frame slightly longer than 10 sec, but cables swapping is not acceptable for me. If I need to track 2-4 sources I'd like to have 4-inputs simultaneously sampling device. I still don't have bench DMM and realy don't like them as all I seen have only one input. Running 2-4 signal in parallel brings time lenght demands down <10 sec again.2. " is not feasible"Can't get a difference 10 LSB & 1 LSB in your head ? Retaired or what ?It;s not about what voltage reference part number is better and what is worse. RC network is always x1000 times lower in noise , x1000.000 times over wideband. There is 2 MHz clock, so overall noise-accumulating bandwith up to 10-th MHz. Even unprecedently low noise like 1nV OPA configured as buffer G=1 active LPF still would output microVolts of noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2023, 12:28:44 pm
content: Some filtering of the reference path absolutely makes sense is standard practice and also shown in the datasheets. This port of the filtering should of cause still be there. The problem is trying to also filter away the low frequency (e.g. 0.1 - 10 Hz) noise. This usually comes with a large resistor and capacitor and this usually leads to worse performance below the filter frequency. The solution with the large capacitor comes with drift and settling issues. The version from the AD article comes with quite some extra 1/f noise.With a DMM and similar more DC measurements the drift and lower frequencies really matter. It is more like the 0.1 - 10 Hz range that one can work around by a little more averaging.Multiple channels and simultaneous sampling can in some cases helps, but not always.I would even consider the part above 1 Hz, where the filter actually works as less important.Trading in the DC and low frequency ( < 0.1 Hz) performance for reduced noise in the 0.1-10 Hz range is usually a bad choice with a DMM.A better reference helps in the whole frequency range and often also comes with better drift specs. It may not be as good as the filter at 10 Hz but the cross over where a better reference can be better than the filter may be at some 1 Hz. The quality of the reference does make a difference - the whole point of the filter is to reduce reference noise and this is especially an issue if one starts with a rather noisy type like the ADC internal one or the max6164. Already a relatively cheap max6070 is expected to be 3 x lower noise - not just in the 0.1-10 Hz band but also below.The test with 128 samples is just in a way to test the range where the filter helps and ignore the low frequencies where it makes things worse.Using the same reference for the input signal and the ADC reference normall eliminats much of the reference noise. It only works good with the same level of filtering on both sides - a more moderate fitler would work just as well. Comparing the case with no filter at the refernce side to the case of the same type of filter for the ref. and input is thus not fair. With even more filtering at the reference it would get worse again.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 01, 2023, 12:30:24 pm
content: Quote from: iMo on June 01, 2023, 07:07:31 amI see. Yes, buffers CM includes Gnd & Vdd. Ether internaly generated -Vcc or buffers is "pre-chargers" and complete voltage sensing cycles includes direct switching - > by pass.Buffers at the reference inputs is important in this topic, since non-linearity is un-avoidable if R in series with Ref.There a few excelences in this ADC (I'm not affiliated in any way and not advertising any product):- byffers;- noise-free channels switching, there is even "delay" option to sample glitch-free. I tried same things with mcp3562 and was disapointed with high noise if two diff channels in useThe only things I don't like, they wire Ready/ Dout on the same pin, creating complication with common SPIQuote from: iMo on June 01, 2023, 10:10:03 amSooner or later it comes to understanding that voltage reference filtering is a must. The problem here, as I already mention, common OPA outputs uVolts - over MHz bandwidth, and to get down to 150nV is AZ OPA configureed with RC network like in a picture or "some tricks" in the ADC at the reference inputs, pre-chargers /AZ etc.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2023, 05:13:44 pm
content: I agree with Kleinstein. Filtering low frequency noise, especially flicker noise, is a fool's errand and without heroic efforts will introduce more low frequency noise, at least in the form of drift. It is better to use a lower noise reference, or multiple references in parallel, to reduce low frequency noise from the source.I settle for the 0.1 to 10 Hz noise measurement because it correlates well with lower frequency measurements and it is much easier to get good results.Quote from: Kleinstein on May 31, 2023, 07:30:09 pmI have assumed, and it has been my experience, that high frequency noise close to the Nyquest frequency will alias to low frequencies compromising low frequency noise, but with the high sampling frequency of a delta-sigma converter, this high frequency noise is easy to remove with moderate filtering and without adding to the low frequency noise.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2023, 05:17:31 pm
content: Quote from: MasterT on June 01, 2023, 12:30:24 pmThat is the circuit I like to use, and not just for reference filtering, because it provides the lowest AC output impedance.There are some modern precision operational amplifiers which support driving the output capacitor directly, which might be as good or better.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 01, 2023, 07:54:01 pm
content: Quote from: David Hess on June 01, 2023, 05:17:31 pmTake a pen and calculate an impedance of the 10 uF capacitor for 10 Hz frequency, or 0.1 Hz Than compare output impedance of the OPA that tolerates capacitance and make a conclusion.The fact is, there is no OPA that can be filttered just setting cap at the output no any voltage references. Output impedance ratio to cap's impedance is enormous, using R-C network is the only way. Test results talk to itself, don't be stupid

author: KT88
author_rank: Frequent Contributor
timestamp: June 01, 2023, 10:07:16 pm
content: There are actually opamps that can drive large caps like 10uF or more. It doesn't mean they can drive them at high frequencies...It's quite the opposite: they drive DC but they won't oscillate.The reference driver and reference cap also don't work as a filter - the cap is a reservoir for charge. The sample action takes some charge out of the ref-cap and the buffer recharges the cap before the next sample is converted. The cap size depends on the allowed error that can be introduced - a rule of thumb is to allow for a voltage drop of 1/2LSB - but that depends, of course...A similar kind of amplifier can be found in more recent LDOs that are specified to operate with larger ceramic caps.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2023, 01:03:32 am
content: Quote from: KT88 on June 01, 2023, 10:07:16 pmQuite a few old operational amplifiers will work that way, including the 741 and OP-07. The large capacitance combined with the output resistance implements dominant mode compensation, with the ESR adding some phase lead so this works better with tantalum or electrolytic capacitors.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2023, 01:07:25 am
content: Quote from: MasterT on June 01, 2023, 07:54:01 pmThe feedback provides a low impedance at low frequencies. The large output capacitance provides low impedance at high frequencies where the open loop gain is lower and the output impedance of the closed loop system is increasing.This configuration is especially useful for rail splitters, and has the advantage of driving difficult loads without instability.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2023, 07:24:57 am
content: The capacitance at the OP-amp output can act as a higher frequency filter, but of cause not as a low frequency filter. Wether calling it filter or buffer is just looking at the same thing in frequency or time domain.Quote from: MasterT on June 01, 2023, 07:54:01 pmThe results shown at the start are a bit misleading:The case with no filter seems to also miss the high frequency filtering for the reference, that is standard and should not be skiped.The case with the low frequency filter is not just about the filter, but about using the same source for the reference and input signal. With the same filtering for both paths reference noise is suppressed. This also works with way less filtering and does not proof the effectiveness of the filter. Test relativ short data window makes the test insensitive to the drift introduced by the filters.The tests are done with a quite noisy reference. The alternative to excessive reference filtering that compromises stability is using a lower noise reference to start with.

author: iMo
author_rank: Super Contributor
timestamp: June 02, 2023, 08:11:56 am
content: The key message here by Kleinstein and others is the applying a large RC filters in the Vref affects the "long term stability" (as the large value capacitors introduce various drifts). Your measurements with your large RC will be less noisy when evaluating small/short set of results (as you did above), but may drift significantly when doing long term measurements.It has no sense to achieve 1LSB peak to peak noise at 1sample/sec (doing 1minute long measurements) with the 1500uF filter capacitor, when it will do ie. 2674LSB p-p drift from day to day or week to week. Thus you may achieve much more reliable results with a noisier Vref chip with small less drifty capacitors in the filter and do averaging/smoothing of the results. Another variant is to use less noisy Vref chip per se.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 02, 2023, 08:55:27 am
content: Quote from: iMo on June 02, 2023, 08:11:56 amLong term stability competely relies on stability of the voltage reference. Period. It has snothing to do with filter."It has no sense to achieve 1LSB peak to peak noise at 1sample/sec" - It does HAVE, this is an objective.I don't give a f. if FPGA-brain-dead retaired does not understand. I show results , it's pure Science - not a "likes-don't likes" vote for AI, opinion does not accounted.

author: iMo
author_rank: Super Contributor
timestamp: June 02, 2023, 09:14:21 am
content: A good test would be, for example:1. a 1 day long measurement with the standard setup (or longer)2. a 1 day long measurement with the large RC filtering (or longer)3. all above done with separate Vref and Vinp sources, the same sampling period4. make ADEV/MADEV of both results to see how it performs short/long term..

author: MasterT
author_rank: Frequent Contributor
timestamp: June 02, 2023, 09:17:57 am
content: Quote from: iMo on June 02, 2023, 09:14:21 amAsk AD to do it for you.10 seconds at max. read above, or get lost

author: Echo88
author_rank: Frequent Contributor
timestamp: June 02, 2023, 10:04:42 am
content: They cant grasp your genius MasterT.While were at it: can you give more hints to the stolen USSR secret sauce that made the 3458A the pinnacle of DVMs like mentioned in the 3458A-AFE-thread or will that lead to forceful defenestration and the end of this thread?

author: MasterT
author_rank: Frequent Contributor
timestamp: June 02, 2023, 10:43:35 am
content: Quote from: Echo88 on June 02, 2023, 10:04:42 amWho care. I know that I made over 200 inventions, that were stollen and than re-flashed to some idiots in the west.It can't be disscused openly, since there are a few NDA:- how my algorith "identify AI" works;- how human brain works;- memory manipulation;- parasitics economy functioning;etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2023, 11:30:01 am
content: Quote from: MasterT on June 02, 2023, 08:55:27 amIdeally that is the case. However a poorly designed filter can add to the drift, e.g. from capacitor leakage current. In the current example this are a claimed 10 nA and 10 K resulting in 100 µV of difference that the filter can make. For the leakage current one can not expect it to be especially stable. So the filter as shown may cause drift issues.

author: dietert1
author_rank: Super Contributor
timestamp: June 02, 2023, 12:27:36 pm
content: Let's remind that the best commercial voltage calibrators use PWM switched references with RC filters. As far as i know this works to about 0.1 ppm.If one ovenizes the filter including its caps, this can also work at long time scales. The caps i tested were film caps, e.g. an AVX FFLI6L0657K cap (650 uF 1000V}. At time constants of 200 to 500 seconds one could see temperature changes as voltage changes: Capacitor at constant charge means voltage inversely proportional to capacitance affected by its TC. One also wants a method to initiate the filter without waiting days. There is no magic about the 10 sec time scale, except temperature will be near constant within 10 seconds, so it can be done without oven.Regards, Dieter

author: MasterT
author_rank: Frequent Contributor
timestamp: June 03, 2023, 04:32:25 am
content: 100 uV accross 10k resistor indeed looks as a big error, but thinking a liitle bit about for a fix I came to solution "zero cost - no external hardware".All it needs to connect pin4 of the ADC to other side of 10 k resistor. Than in software periodicaly, let say 10 sec,( 1 sample out of 100 ) I would reconfigure input MUX matrix to connect (adc-) -> Ref, (adc+) ->pin.4, and voila - real time capacitors leakage current monitoring. Accuracy could be better than 100 nV using LPF in software .Two diff channels based on AIN0-AIN1 & AIN2-AIN3 would run as planned

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2023, 03:59:20 pm
content: Some correction is indeed possible, but one has to be a bit careful. The extra measurements add to the noise - one starts with the noise one wants to filter out and thus needed the digital filtering (LPF) with quite some samples to keep the noise low. So only 1 out of 100 readings for the correction may be on the low side. To get a 10 fold noise reduction over a single reading it would need at least 100 readings to average. To add less than 1/10 to the noise it would be more than that.With the digital filter one may have to be careful with the phase shift delay, not to amplify noise from the transition region of the analog filter. Not sure about that, but there could be some trouble from the delay, as the LP filter naturally delays things and may thus add current noise and past noise that don't fully compensate.Another point is that the ADC input and switching of the mux can be quite some load to the filter and may thus need a buffer to not add extra drift.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 04, 2023, 03:19:39 pm
content: People say "A picture is worth a thousand words".Scale X - 7 min., Y - 2 uV per grid.Red line: 1-st channel diff inputs shorted to track adc offset variation;Blue line: 2-nd channel measuring noise-free 2V;Make your guess what reference for adc was in use.Circuits updated with a 0.1u cap at pin.4

author: MasterT
author_rank: Frequent Contributor
timestamp: June 13, 2023, 02:34:50 pm
content: Updates.Quality of the internal V. Ref of the ad7172 is poor, so I move to external reference.https://www.eevblog.com/forum/chat/lm431-(tl431)-0-1-10-hz-vp-p-noise/new/#newTL431B from OnSemi is my choice, beats up REF5025, REF6141, MAX6164 I tested so far with. Noise tests 0.01-1Hz - based on screen picture over 7 min. interval.Having ~48nV /Hz^-2 it costs only $ 0.5, about 1/50 compare to LTC6655 ($22CAD on mouser).

author: MasterT
author_rank: Frequent Contributor
timestamp: June 15, 2023, 03:12:31 pm
content: Video shows dynamics.ADC is sampling 3 channels, 1 sps /channel, 2 externally connected to filtered TL431B, one is maintenance tracking offset-reference fluctuation.Trends: update rate 1 seconds, data not filtered, as it is, zoom 2 uV/div , time scale 7 min overall.Big digits: update 3 Hz, filtered running average LPF 32 samples.https://youtu.be/aFE5hyFjUE4

author: Kleinstein
author_rank: Super Contributor
timestamp: June 15, 2023, 03:50:14 pm
content: One can clearly see quite some slow drift with the TL431 references, likely from thermal effects.The slightly better references like ref5025 come with a much lower TC and also correction for 2nd order effects in the temperature. This correcton comes with some additional noise, but is needed to get a low TC over a large temperature range, so the reference can be used without temperature control.Good 0.1 ... 10 Hz noise is pretty much useless if the TC is high.It is still impressive how low the noise of this TL431 variant is. It is still not a very useful reference for a low sampling ADC. It could be a good choice for audio though.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 15, 2023, 04:08:54 pm
content: I should say that those up-down drifting caused by me hovering with android over test setup. TL431 installed into breadboard and there is 30 cm du-pont wiring, so I'm not sure if it's thermal effect due IR radiation by my body, or RF interference injected from android device. Taking a picture takes less time, so photo is always less jerky.Even walking less than 1m away from test bench shifting voltage up and down.REF5025 is no difference. can't hold steady voltage if I move hands over it, in my another project I even cover IC by FR4 - with both side metalization, Cu known for good IR/ RF reflection.

author: miro123
author_rank: Frequent Contributor
timestamp: June 17, 2023, 06:30:39 am
content: I did some mid term /90bdays/ test with several TL431 - diffrerent manufacture and batches. Their mid term stability is quite poor. there is workaround over TC but not about humidity and long term stability. I did not made any further investigations. Ihave many cases with unexpected voltage fluctuations. e.g. DUTs are at lab condition continuously powered. lab temperature and humidity curves are similar, measurement are performed early in the morning /less background noise/. I sow frequent spikes in positive and negative direction. Also the ageing slope is quite steep