[sdiy] op amp directly connected to ADC

[rburnett at richieburnett.co.uk]

If nothing else is connected to the op-amp's output, and the op-amp 
doesn't freak out due to the ADC's loading, then the result of the 
sampling glitch is usually one of more of the following:

1. A small scaling error in the measurement
2. A small offset error in the measurement
3. Some degree of non-linearity (harmonic distortion)

How detrimental these effects are depends on the application, what the 
ADC is measuring, and how accurate it needs to be, etc.

-Richie,


On 2019-07-30 17:39, Chris McDowell wrote:
> 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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