0.5 V Digitally-Tunable Filters for Biomedical Applications Exploiting Segmented Duty-Cycled Resistors

In this paper, we present the design and the experimental validation of a biquad filter for biomedical applications that takes advantage of segmented duty-cycled resistors to allow for digital tuning over a wide range of the cutoff frequency (f0), quality factor (Q), and gain (K). Additionally, we report on the experimental findings for an 8th-order band pass filter that was created to extract the epilepsy biomarker, related to the fast ripple band (250 Hz-500 Hz). The results of measurements on a prototype chip, implemented in a commercial 130 nm CMOS technology with a supply voltage of 0.5 V and a power consumption of 120 nW, confirm that the f0of the proposed biquad filter can be digitally tuned from 15 Hz up to 517 Hz, whereas the Q can be tuned from 0.7 up to 13. A comparison against the state of the art has shown that the proposed biquad filter exhibits the lowest area, with the highest Q, and can be tuned at the lowest f0. It also exhibits the unique feature to allow for independent tunability of f0, Q, and K by varying the duty-cycle of digital control signals. The 8th-order filter, implemented with 4 cascaded instances of the proposed biquad filter, guarantees an attenuation higher than 30 dB at f0/2 and 2f0 with a power consumption of 480 nW, and an area footprint of 0.152 mm2, thus satisfying the requirements to extract the epilepsy biomarker in closed-loop deep brain stimulation systems.