[sdiy] CV input op-amp circuit

[Bernard Arthur Hutchins, Jr]

Richie said Dec 4 23:01:12 CET 2020



“ I'd recommend a passive RC lowpass filter between the output of the op-amp and the ADC input. It can serve several purposes:

1. .  .  . . . . 6. “


Reason 7:  A typical anti-aliasing filter is often reflexively supposed to be an even-order active low-pass with an op-amp output feeding directly to the sampler. While the supposed audio bandwidth may be only, say, 20kHz the op-amp’s inherent output noise may extend to several MHz, and once sampled, be FOLDED AND ADDED many times back to the audio (baseband) range.


Accordingly, it is often advisable (as Richie suggests) to have an odd order active filter with the real pole passive and last; and set for low impedance, so that no op-amp buffer is even needed – the RC junction feeding the sampler directly.


Such practice being common with analog delays, CCDs (BBDs).


-Bernie


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