Comparison of demodulation RFI in inverting operational amplifier circuits of the same gain with different input and feedback resistor values

The magnitude of the voltage gain for each circuit is equal to 10. The two op amp circuits are identical with the exception of the values for the input and feedback resistors. In one circuit the value of the input resistor is 10 k Omega , and the value of the feedback resistor is 100 k Omega . For the other circuit, corresponding values for input and feedback resistors are 100 k Omega and 1 M Omega , 50% amplitude modulated (AM) signals were injected into the op amp signal input terminal to produce an undesired demodulated response at the 1 kHz AM-frequency. The RF frequency was varied over the range 0.1 to 400 MHz. Four op amp types were tested. Experimental results show that increasing the values of input and feedback resistors improves the RFI (radio frequency interference) immunity of the inverting op amp circuit. The improvement is caused by the parasitic capacitances of input and feedback resistors.<>