A novel response-translating lowpass filter achieving 1.4-to-15-Hz tunable cutoff for biopotential acquisition systems

This paper presents a response-translation technique to realize an ultra-low-cutoff lowpass filter in small area for biopotential acquisition systems. It is by exploiting a chopper-stabilized instrumentation amplifier (IA) with bandpass characteristic to obtain a lowpass response after chopper stabilization, resulting in substantial area savings because of relaxed time constant in the implementation. Optimized in a 90-nm CMOS process with 2.5-V thick-oxide devices, a 2^nd-order OTA-C ladder filter demonstrates -40 dB/decade stopband attenuation, 1.4-15 Hz frequency tunability, and strong relaxation of the required time constant (depends on the set chopper frequency). The entire lowpass filter draws 8.37 μA at a 3-V supply.