Improved Fully Differential Analog Filters

This paper proposes two novel design techniques that improve the performance of fully differential filters built by coupling two equal single-ended filters. These filters usually reach a high common-mode rejection ratio (CMRR) without using tightly matched passive components but may become unstable because of common-mode signals. Previously proposed techniques to solve these problems, based on resistor or capacitor T-networks, reduce the CMRR. The two techniques proposed overcome this tradeoff. The first technique improves the stability of the filter common-mode gain by adding damping resistors that do not modify the desired differential frequency response or the CMRR. The second technique, which is particularly valuable when a path for input bias currents must be provided, uses a resistor network to estimate the common-mode voltage of the input and uses it as common ground voltage for the filter.