Linear low-frequency filter using on-chip giga-ohm resistance

Integrated grounded resistors of very large value are essential circuit elements for the design of compact filters with very low cut-off frequencies. A typical application is the rejection of DC voltages in amplifier circuits in physiological recording systems exhibiting electrode offset and low-frequency drift. This paper describes the implementation of a giga-ohm resistance using a fixed-gain OTA and a cascade of current scalers. The circuit yields low complexity, small power and area consumption as well as high linearity. Measured results from a test filter integrated in 0.35 μm CMOS technology on an active area of 0.011 mm² are presented. It yields 41 Hz cut-off frequency, 1 V input range and less than -52 dB harmonic distortion.