Electronics > Projects, Designs, and Technical Stuff
Filter caps in diff amps - yes/no
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MagicSmoker:
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.
poorchava:
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.

MagicSmoker:

--- Quote from: poorchava on May 13, 2020, 07:08:16 pm ---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.

--- End quote ---

Right, 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).
jaycee:
How do you know they are of poor tolerance ? They are probably NPO dielectric with 1% tolerance or better, much like the resistors
Someone:

--- Quote from: MagicSmoker on May 13, 2020, 06:48:29 pm ---EEVblog denizens - settle once and for 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 worse the AC CMRR.
--- End quote ---
Depends 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?
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