Analogue circuit design for RF immunity

摘要

Why Bother with immunity design? The answer is because most analogue ICs are susceptible to demodulating radio frequency (RF) signals which are outside their linear frequency range of operation. Unfortunately, there are no data sheet specifications which can act as a predictor for this problem. As a general rule, bipolar op-amps are approximately 20dB less immune than BiFET types. Even "slow" op-amps are sensitive to RF interference on all their pins, regardless of the feedback schemes employed. (See Fig. 1.) All semiconductors demodulate RF signals due to inherent non-linear characteristics. Demodulation is a very common problem in analogue circuits, because they do not have noise thresholds like digital circuits. Contrary to what most low-frequency analogue designers believe, even "slow" op-amps will happily demodulate inter- ference up to cellphone frequencies and beyond, as shown by the real product test results of figure 2 (note that the output error is worse at 1 GHz than at 500MHz). (For more on this issue, read section 4.3.2 of reference 2.)