Modulation/demodulation system for capacitive sensors

A modulation / demodulation system which is suitable for capacitive sensor signal processing is proposed. A capacitive sensor has very weak output signal which requires preamplification prior to any further processing. The unavoidable 1/f noises [2] of the MOSFETs in the electronic circuit often eat away the signal from the sensor, which make the signal not readable. To overcome this problem, a modulation / demodulation system is proposed to modulate the 1/f noise to a higher frequency and then been low-pass filtered. The result shows that the circuit suffers an equivalent input noise of 468nV/rt Hz and has a THD of 0.6% with 5mW power consumption.