Compact, Fully Differential Analog Amplitude Demodulator by Power Supply Voltage Switching

There is a growing interest in developing impedance sensors able to work at low power and with a small footprint. The analog lock-in amplifier (an amplitude-modulated (AM) demodulator) is a common solution to recover the baseband signal from modulating sensors while avoiding low-frequency noise. However, it uses several active components whose total power consumption might shorten battery life. In this work, we propose a simple AM demodulator based on a fully differential switched-gain amplifier. Using op amps with shutdown enables the gain switching between 0 and 1, which recovers the baseband signal in a similar way to a conventional +1/−1 switched gain amplifier, but with a 50% amplitude decrease in the demodulated signal. By using a power-down signal synchronized with the carrier, it is possible to program a 0° or 90° phase that enables in-phase and quadrature demodulation, ultimately allowing the measurement of complex impedances. Tests were performed in two different situations: static and time-varying impedances, and with two different op amp models, the OPA363 and the ADA4806-1. In the former test, several resistors and capacitors were measured, yielding deviations from a reference instrument below 0.5% for resistors and below 2.7% for capacitors when using the OPA363. In the latter test, the electrical bioimpedance changes of the hand-to-hand body segment of a number of healthy volunteers were recorded, enabling the detection of the respiratory and pulse rate.