[sdiy] op amp directly connected to ADC

[Richie Burnett]

I always put an RC filter between the op-amp output and the ADC input.  It 
potentially serves several purposes:

1. The capacitor provides a reservoir of local stored charge for the ADC 
when it's sample & hold capacitor needs to charge up quickly.
2. The resistive part decouples the op-amp's output from the ADC input 
minimising the effect of the sampling on the op-amp output.
3. The resistive part can limit the current through clamping diodes if the 
op-amp output is capable of going beyond the input range of the ADC.
4. The RC filter provides one pole of anti-aliasing filtering for the ADC. 
(This includes stopping all the broadband noise generated by the op-amp from 
aliasing into the ADC's passband.)
5. The resistor limits the resulting fault current if a microcontroller's 
ADC input is accidentally misconfigured as a digital output!

In practice I'm fond of using the MFB filter arrangement for driving ADC 
inputs, so the overall anti-aliasing filter is typically at least 3rd order. 
Two poles (slightly under-damped) from the MFB and a further pole from the 
RC before the signal reaches the ADC, for a 3rd order combined response that 
is close to Butterworth.

With low-speed ADCs and op-amps you can certainly get away without putting 
an RC filter between the op-amp output and the ADC input, but it gets more 
important when you used faster ADC.  For example most multi-megasample RF 
ADCs require a low and controlled impedance driving them if their 
sample-and-hold system is going to settle quickly to within one LSB in the 
tiny time available.  Op-amps typically have a rising output impedance with 
increasing frequency so eventually you have to put some capacitance across 
the input of the ADC to keep the source impedance down.  But most op-amps 
don't like capacitance connected directly across their output and will tend 
to overshoot and ring.  Hence the arrival at a 1st order RC filter.

Another time when it's important to put a capacitor across the ADC input is 
when it's driven directly from a potentiometer.  In this case the source 
impedance varies widely from very low at both ends of the track, to a 
maximum of R/4 in the middle of the track.  The capacitor helps to ensure a 
nice low source impedance for the ADC regardless of pot wiper position.  As 
an aside, the capacitor will also "hold" the previous wiper voltage if the 
wiper momentarily goes over a dirty patch on the track, which is usually 
less objectionable than the ADC input voltage momentarily dropping to zero!

Hope this helps answer your question.

-Richie,


-----Original Message----- 
From: Chris McDowell
Sent: Monday, July 29, 2019 6:39 PM
To: Synth DIY
Subject: [sdiy] op amp directly connected to ADC


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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