A High Linearity Large Time Constants Switched-Resistor Filter for Biomedical Applications

Integrated low-frequency filters for biomedical applications require high resistance values to implement the required very large time constants. In this paper, the Switched-Resistor (S-R) approach is used to realize the proposed low-frequency low-pass filter (LPF) for biomedical applications. The MOSFET switch is included in the feedback loop to further reduce its distortion and increase the filter linearity. A buffer is used to load the feedback resistor, hence eliminating the need for a driver stage, allowing a low-power design. The bandwidth and gain are tunable to handle different biomedical signals. The filter achieves -61.5 dB total harmonic distortion (THD), with a 33 Hz sine 0.14-Vpp differential output. The input-referred noise is 16.2 μVrms, which is mainly contributed by the thermal noise of the input resistors, in a 300 Hz bandwidth. The prototype IC, fabricated in a 0.18-μm CMOS process, occupies an area of 0.08 mm
²
and consumes 3.7 μW from a 1.8-V supply.