An Analog Lock-In Amplifier Based on Synchronous I/Q Demodulation for Capacitance Measurement

In this article, an analog lock-in amplifier (ALIA) based on synchronous in-phase and quadrature (I/Q) demodulation is proposed for wide range capacitance measurement of high accuracy. In order to improve the signal-to-noise ratio (SNR), an extra compensation channel was employed to quickly pre-determine the capacitance to be measured and optimize the measurement configurations. A transimpedance amplifier (TIA) is designed to eliminate the adverse impacts of parasitic parameters for optimal SNR, which is accomplished by applying an adaptive strategy to control the excitation frequency and compensation capacitance. Additionally, a capacitance measurement model with improved accuracy is established by considering the effect of non-ideal characteristics of devices. A prototype is fabricated and tested to verify the effectiveness of the proposed ALIA. It is shown that the ALIA has excellent comprehensive performances, which can achieve capacitance measurement with an average error of 0.52% in a wide range of 10 fF–10 nF. The measurement error is reduced by 9.74% in the full-scale range (FSR) by considering the non-ideal characteristics of devices, which is reduced by 25.24% in measuring small-valued capacitances (<1 pF), compared to a commercial lock-in amplifier (LIA). The ALIA has a strong robustness against additive noise with a SNR of −42 dB. Additionally, a comprehensive uncertainty budget is presented and the uncertainty components of the system are identified and summarized. A low capacitance measurement uncertainty budget ranges from −57.1 to −50.8 dB in the FSR.