This paper presents the design of a CMOS lock-in amplifier (LIA) encapsulated with an impedance sensor for microbial monitoring applications. The custom integrated LIA is designed and fabricated in a 0.18-µm CMOS technology. It includes a fully differential switched-capacitor transimpedance amplifier as the main building block of the lock-in amplifier. In this design, chopper stabilization is used in the capacitive transimpedance amplifier to reduce the noise and improve the sensor\'s sensitivity. The proposed LIA contains a band-pass filter with 0.88 quality factor to pass signals at selectable center frequencies of 1, 2, 4, and 10 kHz; a programmable gain amplifier, a mixer, and a low-pass filter to extract impedance changes caused by microorganism growth at different frequencies. The transimpedance amplifier has a gain of 54.86 dB, and an input-referred noise of 58 pA/√Hz at 1 kHz. The whole sensor has a sensitivity of 240 mV/nA. It consumes a power of 817.56 µW from a 1.8V power supply and has a total harmonic distortion of -72.7 dB.