A micropower class AB CMOS log-domain filter for DECT applications

This paper presents a micropower 2^nd-order low-pass filter using the log-domain principle and integrated in a 0.35 µm CMOS process. It has been designed as an anti-aliasing filter for a DECT transceiver with a 45 kHz nominal cut-off frequency. The circuit uses transistors biased in weak inversion without requiring separate wells. It operates at 1.5V supply voltage and its current consumption is 8 µA in idle mode. The log-filter is implemented with an on-chip conditioner which allows for class AB operation. It can process input currents at 5 kHz that are 25 times larger than the 200 nA nominal current. Measurements up to 500 times the bias current have been done, since at 1 kHz the current input is only limited by strong inversion and by the supply voltage.