Ultra Low Power Low Pass Active CMOS Filter Design For Biomedical Application

Abstract

Ultra low power with biomedical frequency low-pass filters has many applications in sensor interfaces and biomedical signal processing units. Biomedical signals are usually of 10-mHz to 100-Hz frequency range and hence require subhertz frequency filters to condition the signal before processing. The performance of filters designed by the use of passive components degrades at audio frequencies and the required resistances and inductances values calculated from the mathematical expression are very difficult to meet from the market. To find a solution to this problem is a study to realize Passive Filters into Active Filters using Operational Transconductance Amplifier (OTA). For example, the low pass filter using OTA presented here was designed as part of the breathing monitoring device presented in (although it is not restricted to this use). However, despite their utility, creating large time constant low-pass filters on-chip is a challenging problem. To implement large time constants, switched-capacitor-based topologies require large capacitor ratios, and a sufficiently high power supply must be used to achieve an acceptably low switch ON-resistance.

In gm/C-based topologies, the cutoff frequency is generally determined by ratio gm/C, where C is the integrating capacitance present. However, area limitations restrict the maximum capacitance that accomplish here by MIM capacitor on-chip to picofarad values, limiting the minimum cutoff that can be achieved. Larger gm and C values must be used to keep the output current above this limit but at the cost of power consumption. The aim of this work is to determine values of the design parameters that optimize an objective feature whereas satisfying specifications or constraints. This first-order low-pass filter topology is capable of providing cutoff frequencies down to 143 mHz with power consumption of 4.712 nW. The circuit is implemented in a 0.18-μm technology with a 1-V supply In terms of power consumption and cutoff frequency this filter performs better than previous filters from the literature.