A low noise CMOS interface circuit for capacitive liquid crystal chemical and biological sensor

In this paper a low noise CMOS interface circuit has been designed for capacitive liquid crystal (LC) chemical and biological sensors. The interface circuit consists of a low noise differential folded cascode amplifier that has been optimized for noise and performance requirements of the LC sensors. Two on chip interdigitated capacitors with LC on top, provide differential measurement of the sensor change. The differential structure of the measurement system reduces sensitivity to common disturbances like temperature change and parasitic elements. The output signal of the amplifier is demodulated using synchronous demodulation. A nonlinear numerical program has been used for minimization of the total thermal and flicker noise of the amplifier at the specified frequency. To the knowledge of authors this is the first CMOS interface circuit designed for capacitive LC chemical and biological sensor. The main amplifier has 15aF resolution with 1kHz bandwidth and consumes 1mW of power.