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.

I. INTRODUCTION

Natural phenomena signals, such as in medicine, seismology, astrophysics, geophysics, acoustics and vibrations, despite of their growing diversity, remain essentially analog, weak in magnitude, superimposed to high common-mode voltages and usually corrupted by noise. They also share common features, such as frequency components in Hertz/sub-Hertz range. To process such stringent signals, their dc component has to be initially filtered out, which is challenging to accomplish on a fully integrated form due to very-large time constants involved.