[sdiy] CV input op-amp circuit

Richie Burnett rburnett at richieburnett.co.uk
Sat Dec 5 13:38:44 CET 2020

Yes, it stops broadband noise from the op-amp from aliasing over and over 
into the baseband range of the ADC.  I've found this helpful in situations 
where the output of the op-amp contained some unwanted HF switching hash 
that was on the supply rail and made its way to the output of the op-amp due 
to its limited high-frequency Power Supply Rejection Ratio.  (Obviously the 
supply rails should really be clean, or ideally use separate analogue and 
digital power & ground planes.  But sometimes bad stuff happens and a bit of 
switching noise ends up somewhere you didn't want it!)


-----Original Message----- 
From: Bernard Arthur Hutchins, Jr
Sent: Saturday, December 5, 2020 12:37 AM
To: synth-diy at synth-diy.org
Subject: [sdiy] CV input op-amp circuit

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).


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