Analysis of monolithic I/Q based impedance measurement circuits: Impact of non-ideal circuit effects on accuracies

Abstract

I/Q demodulation technique is widely used in monolithic circuits for impedance measurement. The measurement accuracy of impedance is vital for various biomedical applications. The measurement accuracies of the impedance based on the I/Q demodulation was reported around 2%. This paper serves to investigate degradation of measurement accuracy due to non-ideal circuit effects, such as DC components in stimulation currents, offset of amplifiers and frequency-dependent gain in amplifier as well as angular deviation of the I/Q signals. The measurement errors due to the non-ideal effects of practical analog circuits are evaluated mathematically. The errors are quantified in terms of the real part (Er) and imaginary part (Ei) of the impedance. From the results, we gathered that the errors in the real and imaginary parts are both proportional to gain fluctuation. With angular derivation of the I/Q signals from −9° to 9° and an impedance phase difference of 1° to 89°, Er is noted to vary from −6.6% to 4.15%. On the other hand, Ei is noted to range from −896% to 894%. In circuit implementation, the measured Er is around 1%. As for the imagery part, the measurement accuracy suffers greatly, even up to 109% with just 1° angular deviation of the I/Q signals. The findings here are beneficial for impedance measurement circuitry design, providing a clear instruction on how to enhance the measurement accuracies.