Low-Frequency CMOS Bandpass Filter for PIR Sensors in Wireless Sensor Nodes

In this paper, a CMOS fourth-order low-frequency bandpass filter for passive pyroelectric infrared sensors is presented. The sensor is intended for use in wireless sensor nodes, demanding strict low power requirements. The final use of these sensor nodes is an ambient assisted living system for elderly people living alone at home. A NICERA RE200B passive pyroelectric infrared sensor with a measured steady operation current of 3.5 μA has been used. The filter has been implemented cascading two biquad OTA-C filtering stages. OTA-C topology has been selected due to the extremely large time constant of the filter. The second-order stage circuit has been prototyped in a 0.35-μm CMOS process and power consumption is below 6.5 μW from a 3 V supply. A central frequency of 1.49 Hz, with Q = 0.5, and a gain of 45 V/V was achieved.