A Tunable 3rd – OrderLow–PassFilter for ECG Acquisition Using Capacitance Multiplier

Abstract

This paper presents a pseudodifferential third-order low-pass filter suitable for electrocardiography (ECG) acquisition. The implementation is based on a three real poles topology, where unity gain buffers were used for decoupling. Since from the application perspective (low-pass bandwidth), very large time constants are needed for an on-chip solution, the time constants are obtained by using the output resistance of unity gain buffer, where the transistors are biased in weak inversion and loaded by a capacitance multiplier. Based on the possibility of electronic tunability of a capacitance multiplier, a variable corner frequency over two decades (up to kHz) is obtained. The filter prototype was implemented using 180-nm AMS technology. It is shown that for the nominal case, a cut-off frequency of 250 Hz was obtained using a power consumption of 89 nA from a power supply of 1.8 V, for a pseudodifferential solution, and a figure of merit of 0.6 x 10-15 J. Over global variation, for a 50 Hz input frequency and 0.2 Vp-p amplitude, the filter achieves a total harmonic distortions (THD) bellow -58dB.