Employing R-0.5R networks in ultra-low bio-medical active-RC lowpass filters

Utilizing R-2R ladders in design of active-RC approach promotes IC implementation of low frequency filters and leads to high linear and low power design solution for biomedical applications. However, area of R-2R ladders limits low frequency operation of this approach to few tens of Hz. This range is sufficient for designing 60 Hz powerline notch filter, but biomedical lowpass filers requires much lower pole frequencies. This paper investigates the use of other types of ladders (R-αR) in filter design. It shows that among various R-αR ladders, the R-0.5R ladders provide the most attractive solution for designing lowpass filters with ultra-low pole frequency. It is shown that R-0.5R ladders would require as low as 0.7 % of the area of R-2R ladders for realizing a given pole frequency. A second-order fully differential filter was fabricated in a standard 0.18 µm CMOS process. Experimental results showing cutoff frequency of around 1 Hz are provided.