Filter caps in diff amps - yes/no - Page 1
Forum section: electronics / projects

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 13, 2020, 06:48:29 pm
content: EEVblog denizens - settle once and for all whether frequency-response-tailoring capacitors should be used in the standard single op-amp differential amplifier circuit (circled - ellipsed? - in blue in the attached schematic excerpt). My position is that neither capacitor should be used because capacitor tolerances are rather poor and any imbalance in their values will worsen the AC CMRR.P.S. - Feel free to comment on the 1R resistor in series with the Vdd2 pin as well.EDIT - added the "all" after "once and for" above... poor editing on my part in the first place.EDIT 2 - worse -> worsen... "sorry my english" applies here, I guess.

author: poorchava
author_rank: Super Contributor
timestamp: May 13, 2020, 07:08:16 pm
content: One situation where I find them necessary is when measuring rather high-ish (10s of kHz) frequency high voltage signals. A diff amp is needed with high input resistance (>1M), sometimes because of necessary division ratio and isolation/safety. Unless you add those capacitors in question you'll end up with nonsense input compensation cap values.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 13, 2020, 07:36:43 pm
content: Quote from: poorchava on May 13, 2020, 07:08:16 pmRight, like in a differential scope probe, but in that case you often need a variable capacitor somewhere to tweak the frequency response to deal with the inevitable imbalance in strays (if nothing else).

author: jaycee
author_rank: Regular Contributor
timestamp: May 13, 2020, 11:25:40 pm
content: How do you know they are of poor tolerance ? They are probably NPO dielectric with 1% tolerance or better, much like the resistors

author: Someone
author_rank: Super Contributor
timestamp: May 13, 2020, 11:36:17 pm
content: Quote from: MagicSmoker on May 13, 2020, 06:48:29 pmDepends what you are trying to improve and balance, the CMRR as a source of unwanted signal, compared to the unwanted high frequency content (aliasing into the measurement band etc).The tolerance of the capacitors may not be the biggest contributor. Whats the output impedance of the opamp? How balanced are the impedances seen by the two input paths then? How does that compare to the tolerances of the passives?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: May 13, 2020, 11:45:03 pm
content: Since the AC CMRR is reduced due to impedance mismatch created by the cap and resistor in parallel, ( so any CMRR reduction is frequency dependent) starting at no effect at DC and slowly increasing to it's max at the corner frequency .. If those caps are 1% then at a decade below the -3dB those caps would have an equivalent mismatch effect of those .1% tol resistors, then increasing effect peaking at the -3dB and then above fc CMRR is increasing again as the gain rolls off. The designer decided the small reduction in CMRR over that decade is not a problem, I would probably agree.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2020, 03:44:39 am
content: How much does it actually matter? Keep in mind the CMRR goes to pot up near the amp's GBW anyway, so you aren't worsening it much with crummy caps.Where high CMRR is needed, including through cutoff, a more precise filter is called for (and probably an amp with higher GBW), perhaps incorporating common mode chokes (which can have better balance than pairs of capacitors). If you also need high bandwidth and impedance (such that is impractical to achieve with much or any filtering), good luck...Tim

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 10:17:44 am
content: Quote from: MagicSmoker on May 13, 2020, 06:48:29 pmThey should not be used for tailor the bandwidth for the reason you identify and the same goes for capacitors shunting the input. They may be required to suppress RFI or to cancel input capacitance but these occur at much higher frequencies. This also applies for high pass filtering except to provide AC coupling.If the bandwidth of the signal is to be tailored, then it should be done after the output. The not so obvious way, especially for a high pass response, is to apply the filtered output to the instrumentation amplifier's reference input.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 14, 2020, 01:35:53 pm
content: Thanks for the replies - I'll respond to all of them here.Quote from: jaycee on May 13, 2020, 11:25:40 pmThe 270pF caps are 5% tolerance NP0 (not specified in the excerpt, I see), so 50x worse than the 0.1% tolerance resistors (which is specified...), but even if the caps were 1% tolerance that still counts as relatively poor in comparison.***Quote from: Someone on May 13, 2020, 11:36:17 pmSure, other things can contribute to AC CMRR degradation, but in this specific case I am only interested in the relative merits (or lack thereof) of slapping some 5% capacitors across 0.1% gain-setting resistors.***Quote from: Kevin.D on May 13, 2020, 11:45:03 pmYep, that's my argument.Quote from: Kevin.D on May 13, 2020, 11:45:03 pmOkay, one might argue that the reduction in CMRR from capacitance mismatch is not much of a problem (keeping with your assumption of 1% caps, as the argument is rather less credible when 5% caps are used), but what do you gain in the bargain? My argument is nothing. Tailoring the frequency response should happen on the single-ended output of the diff amp, not in the differential input, simply because it is much more difficult to match the reactances of each arm compared to the resistances (if it wasn't then HV differential scope probes with high bandwidth wouldn't cost so much!).***Quote from: T3sl4co1l on May 14, 2020, 03:44:39 amA fair question - does it actually matter - but see above response to Kevin.D. And given that 270pF + 24.9k results in a pole at 24kHz, it would seem to me that an imbalance in capacitor values would start degrading CMRR well before the unity gain bandwidth of even the lowly 741 or 324.Quote from: T3sl4co1l on May 14, 2020, 03:44:39 amWell, clearly a high input impedance diff amp isn't needed since the signal source (admittedly only partially shown) is itself an amplifier with a differential output, and also not mentioned is that there is significant common mode noise zipping around inside this box at 50kHz, which the chosen op-amp still has an intrinsic CMRR of around 50dB, according to the datasheet. The goal here is to reject noise at the 50kHz switching frequency of the main power converters, rather than, say, try to maintain gain flatness at >10MHz or so.***Quote from: David Hess on May 14, 2020, 10:17:44 amYep, I've always applied filtering after any differential-to-single-ended conversion, whether using either a single op-amp or a true instrumentation amp to perform the latter. Admittedly, I didn't give this much thought until the current situation popped up, but it - and an earlier reply from T3sl4co1l about a current sense amplifier circuit I am fond of using - caused me to rethink (or think about) some long-held assumptions. Thanks for the replies, everyone; I think I'll do a treez and give you all a thanks for participating...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2020, 02:15:02 pm
content: Heh, which makes it even worse in a sense, as fully differential amps tend to perform well with CMRR. At least, this was the case the last time I was looking at them, which was the faster (low GHz GBW) kind.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 14, 2020, 02:23:19 pm
content: Good points made in this thread. My personal take on this would be: no. If you need any filtering, as others have said, do this after the differential amp stage.For RFI/EMI filtering, you may in some cases also do this before the diff amp (with common mode filters of some kind for instance.)Although specifically dealing with true in-amps, you can take a look at this note:https://www.analog.com/media/en/training-seminars/tutorials/MT-070.pdfIn any cases, I would not do this in the feedback network. Don't think it's worth the trouble.

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 02:54:17 pm
content: In applications where the highest AC common mode rejection is required, a variable trimmer capacitor, sometimes a butterfly capacitor with independent stators, is used. But this is *not* used for bandwidth control because the adjustment range of variable capacitors is too small.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 14, 2020, 09:56:32 pm
content: So I was thinking about this recently. I have a case where filters are used not in the feeback loop but at the input to a differential amplifier. The justification was that the input is a high speed (~GS/s) DAC with return-to-zero output. So the sample pulses from the DAC far exceed the slew rate and bandwidth of the amplifier. So the concern is that if you don't filter the signal before the amplifier you will increase amplifier distortion even in the signal band you care about. However, I am not sure this argument hold up.