Widely Tunable Bandwidth Ultra Low Power Continuous Time Filters for Biomedical Applications

Abstract

This research intends to focus on filter architectures for bio-medical applications. The proposed filter architectures are designed to operate on a supply voltage of 1.8 V in 0.18 µm standard CMOS process from UMC technologies. Initially, this thesis proposes a novel architecture for realizing very low transconductance values with tunability. This transconductor architecture makes it possible to realize a fully differential filter without the need of explicit common-mode feed back (CMFB) circuit. Using the proposed transconductor a fourth-order, low pass Butterworth gm-C filter with tunable bandwidth has been designed for biomedical signal processing frontends. This novel filter architecure has two bandwidth tuning schemes, a resistor based tuning (R-tuning) and a switched transconductor based tuning (D-tuning). With these tuning techniques, the filter is made tunable in the range 30 mHz-100 Hz. The filter operating on 1.8 V supply is found to offer a DR of 56 dB consuming a power of 1.14 µW. In terms of Figure-of-Merit (FoM), the proposed filter is found to be on par with the filters reported in literature. This thesis also proposes a second-order low pass Butterworth filter with tunable bandwidth capable of offering a dynamic range of 91.86 dB. The proposed filter is based on a sub-threshold source follower. The main idea is to exploit the strengths of sub-threshold source follower circuit, like low noise, low output impedance, high linearity and low power. The transistor bias currents are switched to enable the bandwidth tuning in the range 4 Hz - 100 Hz. A PTAT current reference circuit helps to keep the bandwidth intact across process, voltage and temperature variations. In terms of noise and dynamic range the reported filter is better than previous works found from the literature. The filter consumes 25.9 nW making it a potential candidate for portable biomedical applications. Finally, this thesis proposes a scheme to enhance the output resistance of a differential amplifier. A negative resistance is used to cancel the output resistance of the differential amplifier. This inturn is achieved through a transimpedance based loop. It is proved that the proposed scheme can vgive a gain enhancement of about two folds (in dB) compared to the gain of basic differential transconductor, without loss in linearity. An OTA designed using this scheme is found to offer least sensitivity of gain boost over output voltage swing across process corners, at nominal voltage and temperature, when compared to other methods found in literature. A first order filter is designed using the proposed ro enhanced OTA. The filter offer cut-off frequency down to 0.5 Hz without any switching scheme, and more significantly without the loss of DC gain.