Controllable limiter of signal amplitudes for bioimpedance measurements

Abstract

In electrical bioimpedance (EBI) measurements, the amplitude of excitation signals is strictly kept within certain limits by several circumstances. This is a reason, why also the signal-to-noise ratio (SNR) becomes low that increases the probability of saturation of measurement channels by noise, leading to nonlinearity and signal cutoffs this way. Moreover, larger disturbances may damage the circuitry intended for low -level signals. To avoid this, keeping the amplitude of a sum of signal-plus-noise within certain limits is required. However, due to the low level of signals and the sensitivity to stray impedances (mainly capacitances), the standard limiter circuits are not always applicable in EBI measurements. A typical case is an input of the current-to-voltage converter (transimpedance amplifier). Moreover, the required limiting levels should be low but accurate and adjustable, typically. The introduced novel limiter circuit efficiently protects the input of the transimpedance amplifier (TIA) from overloading and avoids saturation of its output stages. It allows adjustment of limiting levels of the output signal down to the voltage drop of a single diode (0.6 V typically) and has a minimal impact on the TIA properties above this level. However, limiting of the signal amplitude has still considerable impact on the accuracy of measurement channels. The caused errors are measured and calculated for different SNR values. The results described here could be useful also in other implementations where limiting of low-level signals is required.