Charge drift, modulators

Since capacitors do not pass dc current, special steps must be taken to set the charge stored on each capacitor and protect the circuit from drift due to leakage currents, carrier generation, etc. The resistors in Fig. 6.1.8 labelled R s serve this purpose. Additionally, measurements are usually not performed at dc, but rather at some carrier frequency /m, typically a few 100 kHz or MHz. The higher frequencies are needed especially with CMOS circuits to reject the electronics own low frequency flicker noise which often extends to the low MHz. The only exception are applications in which the signal itself is modulated or free from dc content, such as resonant gyroscopes [1] or microphones. Even in those cases modulation is often used. 

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