[sdiy] op amp directly connected to ADC

Vladimir Pantelic vladoman at gmail.com
Mon Jul 29 19:52:23 CEST 2019


my understanding is that you want to keep the thing driving the ADC input as low 
impedance as possible in order to minimize the time to charge the internal S&H 
cap in the ADC, esp if you are scanning several channels using the internal muxer.

if you put an RC you need to take it's time constant into account when 
calculating the needed sampling time of the ADC...


On 29/07/2019 19:39, Chris McDowell wrote:
> I have often seen the output of op amps connected directly to ADC pins on an 
> MCU. Mutable modules all do this, and they perform well and are respected. I've 
> recently been experiencing (then reading about) why this may not always be a 
> good idea, and I wonder 1: what is the best practice here (I believe the answer 
> is an RC after the opamp…) and 2: when can we get away without that RC?
> 
> I'm really asking because I've never had -practical- problems with this. My 
> employer at my day job, though, has decided that because of the noise induced by 
> the op amp sourcing the fast gulps of current to charge the sampling capacitor 
> in the ADC, that we will always put an R between op amp output and ADC input. 
> This bugs me, as it then often necessitates the C to keep our sampling freq up, 
> and two extra parts adds up quick on cramped and cheap designs. I guess another 
> good question is: am I wrong about this? hah ¯\_(ツ)_/¯
> 
> 
> *Chris McDowell*
> 
> 
> 
> 
> 
> 
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