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Technical Tidbit - November 2013
Analog Designer's Notebook Part 1 - Improving Opamp RF Immunity

Non-inverting opamp circuit with RFI improvements

Figure 1.
Non-inverting Opamp Circuit with RFI Improvement Capacitor, CRFI

Abstract: Radio frequency (RF) energy can cause DC offset and noise in opamp circuits, even voice frequency circuits. The RF energy may originate from external sources or even be high frequency noise from digital circuits on the same PCB. A simple way of improving the RF immunity of an opamp circuit that can help in many cases is described as well as a precaution that should be taken when this technique is used.

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, CRFI 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 CRFI 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.

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Copyright 2013 Douglas C. Smith