In addition to the RF sources that
immediately come to mind, such as cell phones and radio transmitters,
opamp circuits can sometimes be affected by digital noise on the PCB,
or even a nearby PCB. One would think that voice frequency opamps would
not respond to RF noise since it is much higher in frequency than the
unity gain frequency of the opamp. However, voice frequency opamps can
act like crystal radios and generate audible noise in the baseband from
logic noise. Voice frequency circuits are sometimes implemented on two
layer PCBs and, for this case, such opamps have been known to be
sensitive to digital noise on nearby PCBs.
One fix for this problem that I have used with success is to put a small capacitor, C
RFI
in Figure 1. across the + and - inputs of the opamp. This capacitor is
small, with a typical value of 47 pF or smaller for feedback resistors
on the order of a few tens of thousands of Ohms, and can result in
substantial improvement in RFI immunity.
One caution, adding C
RFI can reduce the gain and phase
margin of the circuit. Before using this technique, be sure to measure,
calculate, or simulate the effect of the added capacitor on gain and
phase margin of the circuit to make sure a marginal instability does
not result over variations in opamp characteristics over time and
production runs.
Summary: Adding a
small capacitor, usually a few tens of picofarads, across the + and -
inputs of an opamp can improve its RFI immunity, even for voice
frequency opamps, with the caveat that the effect of the capacitor on
the gain and phase margin of the circuit should be checked.
