Fully integrated charge sensitive amplifier for readout of micromechanical capacitive sensors

This paper presents a charge sensitive amplifier (CSA) for readout of micromechanical capacitive sensors. The transfer function and noise performance together with noise optimisation are studied from a theoretical point of view. The feedback resistors of the CSA are implemented as long-channel MOS transistors to set the -3 dB corner frequency well below 1 kHz. The common-mode feedback is designed to keep the input common-mode level constant to achieve an accurate gain from the change in capacitance to output signal. A CSA was designed and implemented for the readout of a bulk micromachined gyroscope with a resonance frequency of 8-10 kHz and signal bandwidth of 100 Hz. A measured resolution of 2.47-3.00 aF (rms) in capacitance change integrated over the signal band is achieved. The CSA tolerates leakage currents of over 5 nA at its input.