A low power tuning scheme for low frequency continuous time filters

In this paper, a tuning technique based on magnitude comparison is proposed for the first time for a composite source follower based second order low pass filter. This technique is targeted for continuous time Gm-C filters for low-frequency and low-power applications such as biomedical instruments. It uses direct tuning technique. The core tuning circuit consists of comparator, up-down counter, current steering DAC (CSDAC) and logic gates. The filter output across the integrating capacitor is compared with reference voltage over several cycles. Based on the comparator command, up-down counter is activated to adjust the tuning voltage of the CSDAC until the reference is reached. The complete tuning circuit and the filter core are designed and implemented in UMC-0.18 ¿m CMOS process with 1.0 V supply. The desired cutoff frequency of 50 Hz is obtained against PVT variations with an accuracy of 2%. The power consumption of the tuning circuit is 54 ¿W, whereas the filter core consumes 6 nW. The total capacitance used is 50 pF. This technique is also applicable for current mode filters.