Lock-In Amplifier for Sensor Application Using Second Order Harmonic Frequency With Automatic Background Phase Calibration

Abstract

An integrated lock-in amplifier (LIA) with automatic phase tuning and second-order harmonic frequency extraction is presented in this paper. The automatic phase alignment loop implemented in the proposed single-channel LIA can directly align the phases of an input signal with a reference signal. Furthermore, the proposed LIA system also includes an enhanced signal detection method in which the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2^{\text {nd}}$ </tex-math></inline-formula> harmonics of the carrier frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\omega _{\textit {ref}}$ </tex-math></inline-formula> ) is applied. As opposed to conventional LIA, this method reduces flicker noise and input offset effects that limit the signal-to-noise ratio (SNR). In terms of signal power detection, the proposed single-channel LIA architecture does not require phase synchronization between input and reference signals. The circuit is designed for a reference frequency of 100 Hz, suitable for biomedical applications. The LIA is Implemented using 0.18- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS technology with a single power supply voltage of 1.8 V. The system consumes 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> at an operating frequency of 100 Hz and presents a high input sensitivity dynamic range for a detection bandwidth of 50 Hz and a FOM of 116 dB.