Ultra low power PVT independent sub-threshold gm-C filters for low frequency biomedical applications

An ultra low power gm-C filter for low cut-off frequency bio-medical implantable applications is presented in this paper. The design is based on a novel trans-conductance stage operating in the sub-threshold region with a PVT independent fixed replica gm biasing circuit. The significant variation in gm with respect to temperature due to channel length modulation is corrected using a parallel combination of two gm blocks of different value. The circuit performance remains nearly constant for a supply voltage range of 1.1-1.7 V. A general biquad filter has been implemented and simulations show about 1% variation in the 3-dB bandwidth (in the range of few hertz) for a temperature range of -30 to 110 degrees C. The circuit has been implemented using 0.5 mu m CMOS technology with a nominal supply voltage of 1.2 V and power consumption for each gm block is 55.3 nW.