An Integrated Analog Lock-In Amplifier using a Passive 3-Path Band-Pass Filter for a Fluxgate Sensor Readout Circuit

Lock-in amplifiers are versatile measuring instruments for extracting a narrow-bandwidth signal with a known carrier from noisy environments. The lock-in amplifier presented here has therefore been designed to provide an efficient readout of the second harmonic of fluxgate sensors. However, existing analog lock-in amplifiers use band-pass filters with increased component count or even omit them altogether. Therefore, the proposed lock-in amplifier takes advantage of the N-path filter principle to minimize chip area and power consumption. The lock-in amplifier consists of a low noise programmable gain preamplifier and a passive band-pass filter that uses a switched-capacitor topology. In this way, the filter achieves a high level of power efficiency and reconfigurability in terms of a tunable center frequency and bandwidth. The complete circuit is integrated on chip using a 180nm CMOS technology and occupies a total area of only 0.42mm$^{2}$. Moreover, the lock-in amplifier achieves a simulated input-referred noise voltage of 5.5nV /$\sqrt{Hz}$ at 10kHz while consuming less than 1mA under a 1.8V supply voltage.