A Programmable Subthreshold Capacitance Scaler for Medical Signal Conditioning

Noise and artifact rejection is a key requirement in biomedical engineering applications. The power and voltage efficient implementation of frequency selective analog front ends is conditioned by supporting large time constants while consuming very little power. MOS transistors biased in weak inversion have been successfully employed to design Gm-C filters. One solution proposed in the literature for realizing large time constants is to use capacitance multipliers at the expense of some additional power consumption. The parasitic series and parallel resistances of scaled capacitors can significantly degrade the frequency range and parameters of integrator cells. This paper proposes an improved architecture for a wide frequency range, programmable current mirror based capacitance multiplier featuring reduced sensitivity to changing programming bits. The enhanced circuit is using a folded cascode current buffer to isolate the programmable current mirror output stage from the multiplier input nodes and to obtain lower sensitivities with the programmed multiplication ratio.