Distributed switched-resistor approach for high-Q biquad filters

Abstract

The switched-resistor (S-R) approach is becoming more and more popular among integrated circuits designers because it allows the implementation of very high equivalent resistances in CMOS circuits. When using this technique, the value of a reference poly resistor is multiplied by a factor dependent on the duty cycle of the clock signal. To achieve resistance multiplication factors higher than 100, the value of the duty cycle has to be reduced below 0.01, but in this case the effects of parasitic components are not negligible anymore and the value of the equivalent resistance tends to saturate at a maximum value. In this paper, we present a distributed switched-resistor approach that aims to strongly mitigate the effect of parasitic capacitances on the value of the equivalent resistance even when exploiting very small values for the duty cycle, thus allowing resistance multiplication factors up to a few thousands. As a validation of this approach, we consider the implementation of a high Q biquad filter for neural recording applications and compare the conventional S-R technique against the proposed distributed S-R approach in terms of the maximum achievable Q factor.