[sdiy] CV input op-amp circuit

Richie Burnett rburnett at richieburnett.co.uk
Fri 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. The resistor limits current into the ADC's clamping diodes if the op-amp output tries to go beyond the acceptable input range as Tom mentioned.

2. The RC filter provides one of the poles of your anti-aliasing filtering ahead of the ADC.

3. The C provides a stiff source of stored charge local to the ADC for its internal sample & hold capacitance to charge from.  (This can yield more accurate conversion results than driving directly from the op-amp if it's output impedance rises at high frequency.)

4. The filter buffers any current pulses caused by the ADC's multiplexing/sampling circuit preventing them from introducing glitches on the op-amp's output in cases where the analogue signal is also used elsewhere and needs to be clean.

5. The resistor limits current flow if you're using a micro with configurable pins and you accidentally set the pin as a digital output instead of an ADC input!

6. In cases where the ADC input isn't fed from an op-amp but from the outside world, the capacitance across the ADC input provides some ESD protection.


---- Tom Wiltshire wrote ----

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