[sdiy] op amp directly connected to ADC

Chris McDowell declareupdate at gmail.com
Tue Jul 30 18:39:06 CEST 2019


Thanks, Richie! Exactly the insight I was hoping for here :) 

So, I guess folks like mutable get away without the RC because the spike induced on the op amp's output is shorter than the acquisition time? And they're just not that worried about it? maybe just the latter, heh. 

Chris McDowell





> On Jul 29, 2019, at 2:06 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 
> 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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