Design of limiting/logarithmic amplifier for wideband bioimpedance measuring devices

In this paper, a limiting/logarithmic amplifier (LLA) for high dynamic range wideband bioelectrical impedance measurements is presented. The amplifier is composed of eight cascaded gain stages with a folded diode-connected transistor as a load that attain wide bandwidth performance with limited power consumption. The logarithmic conversion of the input variable is carried out with the aid of nine detectors. A prototype in standard 0.35-μm CMOS technology occupies 0.06 mm² of silicon area and dissipates 2.2 mW from a single 2-V supply. Post-layout results show that the LLA is capable of processing a 65-dB input dynamic range over a frequency interval from 1 kHz to 1 MHz with an accuracy within ± 0.7 dB.