Lock-in amplifier impedance meter using a low-cost microcontroller

This paper presents a new approach to the classical design and implementation of a Lock-In Amplifier (LIA) applied to impedance measurement of resistances, capacitances, inductors and their arrangements. It is based on a general-purpose microcontroller and an active band-pass filter. Instead of using a phase sensitive detector (PSD), the proposed LIA uses subtraction between samples to recover in-phase and in-quadrature. The design uses a low frequency Pulse Width Modulation (PWM) as source for the reference signal and an Analog to Digital Converter (ADC) from the microcontroller to sample the device under test (DUT) signal. In the particular context of this study, the results are limited to discrete components with relative error around 10%, which rises with higher impedance magnitudes. The measurement error pattern observed in RC, RL and LCR is also presented in this paper in order to emulate practical applications.