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

Abstract

Demodulation RFI responses in two inverting operational amplifier (op amp) circuits were measured. 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 input resistor is equal to 10 kQ, and the value of feedback resistor is equal to 100 kQ. For the other circuit, corresponding values for input and feedback resistors are 100 kQ and 1 M Q . 50% Amplitude-Modulated ( A M ) signals were injected into the op amp signal input terminal to produce an undesired demodulated response at the 1 kHz AMfrequency. The RF frequency was varied over the range 0.1 to 400 MHz. The demodulation RFI was characterized by a nonlinear transfer function H2. The op amp types tested were the 741, OP27, LM10, CA081, and LF355. A team of undergraduate Electrical Engineering students used a semi-automatic data acquisition system to measure the nonlinear demodulation responses and the linear voltage gain responses of the five op amp types. Each student was assigned 25 or 30 op amps of one of the op amp types. The experimental results show that increasing the values of input and feedback resistors improves the RFI immunity of the inverting op amp circuit. For the circuit with higher resistor values the mean of H2 values were decreased 2 to 31 dB for the 7 4 1 , 5 to 41 dB for the OP27, 4 to 33 dB for the LM10, 4 to 35 dB for the CA081, and 4 to 39 dB for the LF355 at a specific frequency over the frequency range 0.6 MHz to 400 HHz. The improvement is caused by the parasitic capacitances of input and feedback resistors.

Taxonomy and Challenges of Out-of-Band Signal Injection Attacks and Defenses

“…A similar methodology can be applied to evaluate the demodulation characteristics of amplifiers and transistors [56], [57], [60], [66], and therefore better predict the fidelity with which attackers can inject target waveforms, both in the conducted and in the radiated settings. This was recently done by Giechaskiel et al [44] for six ADCs, with a view on how to exploit the demodulating effect for out-of-band signal injection attacks.…”