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

Abstract

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 ( $f_{0}$ ), 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 $f_{0}$ of 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 $f_{0}$ . It also exhibits the unique feature to allow for independent tunability of $f_{0}$ , 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 $f_{0}$ /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.

Keywords

• Low-voltage
• active filters
• biomedical applications
• large time constants
• integrated circuits
• switched-resistor