Beware of Loop Gain - John Dunn, Consultant, Ambertec, P.E., P.C.

It is sometimes suggested that an op-amp can be made less susceptible to electromagnetic interference (EMI) by connecting a capacitance directly between the op-amp's non-inverting and inverting input pins, but doing so is something I very much counsel against.

Here's one example why:  

 

Some years ago, we had a seminar here on Long Island delivered by Bob Pease, then of National Semiconductor, where he suggested using such a capacitor, recommending a value of 1000 pF. I raised the question of such a capacitor possibly causing feedback instability due to loss of phase margin.

"I'm glad you asked that question!", he replied. Then he lowered the 1000 pF suggestion down to 100 pF.

Even then, I would still not like to do this. It is preferable to find ways to keep the EMI away from the op-amp altogether than trying to render that op-amp immune to incoming EMI's effects.

Comments

[george storm]

I agree that this is almost always a third-best way of dealing with interference - even though any ringing or instability can nearly always be corrected - either via a small capacitor in parallel with the feedback resistor or a small resistor in series with the filter capacitor (the value of either of which is easily calculated).

Apart from any other reasons, the method as proposed does nothing to reduce the common-mode component - and not many OPAMPs are designed to be tolerant to common mode signals that lie outside the operating frequency range.

[Vladimir Doubovis]

The capacitor C3 together with resistor R3 is working as the integrator that creates delay in feedback loop, so higher T=R3*C3 should create higher instability. It is matching to your pictures instability levels for 100K and 40k.
Ideally this capacitor should not be used, excluding a case when you need to boost gain at high frequencies.

[Roy Laurnet]

I use RC decoupling to the power pins and on the PCB, I keep the inputs as short as possible to help prevent EMI issues. When dealing with extremely sensitive circuitry, I use the "mecca grounding" technique for adjoining stages. Keeps thing mighty quiet and fairly free from EMI too.

[Atul]

a care full analysis helps on use of C3 cap. It causes effect on ac gain and reduced phase margin. So some one need to analyse the trade off between immunity needs vs output ringing and gain stability