Low-power lock-in amplifier for complex impedance measurement

This paper presents a fully integrated lock-in amplifier intended for nanowire gas sensing. The nanowire will change its conductivity according to the concentration of an absorbing gas. To ensure accurate nanowire impedance measurement, the lock-in technique is implemented to attenuate the low frequency noise and offset by synchronous demodulation or phase-sensitive detection (PSD). Dual-channel lock-in amplifier also provides both resistive and capacitive information of the nanowire in separate channels. Measurement results of the resistors and capacitors show 2% resolution over 10 KΩ - 40 KΩ resistance and 3% resolution over 0.5 nF-1.8 nF capacitance. Moreover, a palladium nanowire sensor was placed in a sealed chamber filled with 10% hydrogen in nitrogen, a 28.7 KΩ to 32.1 KΩ impedance variation was measured through the lock-in amplifier by slowly decreasing the hydrogen concentration. The chip has been implemented with UMC 0.18μm CMOS technology and occupies an area of 2 mm². The power consumption of the readout circuit is 2 mW from a 1.8V supply.