A fully integrated high-pass filter for very low frequency applications

A fully-integrated high-pass CMOS filter with Hertz/sub-Hertz break frequencies is presented. The new common-mode voltage is set by a simple feedback loop comprising a servo-amplifier and two common-drain transistors. No bulky resistors are required, making the approach compact and fully integrated. Since the input current is limited to tens of pA, the filter equivalent input-resistance reaches GOmega-range, whereas demanding relatively small layout area. The circuit was designed according to a 0.35 mum CMOS process and 1.5 V-supply. For a 33 pF-coupling capacitor, the simulated cut-off frequency varies from 0.085 Hz to 1.55 Hz, so that the AC-coupler emulates an equivalent resistor of 3.1 GOmega @ 85degC. THD is -72 dB and SNR= 89.2 dB for a 300 mV-amplitude signal. Filter consumption is below 1.05 muW.