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 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 00 or 900 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 was recorded, enabling the detection of the respiratory and pulse rates.