Low Power Integrated Continuous-Time Transconductance-Capacitor Filters Targeted to Operate On 0.5 V Supply Voltage

Abstract

This research work presents continuous-time Gm-C low-pass filters in 0.18 µm standard CMOS process for operation on 0.5 V supply. The filters use bulk-driven transconductors as their building blocks. Two filters are designed to validate the proposed ideas. One is a fourth order low-pass filter offering Butterworth response with a bandwidth of 1 MHz. This filter uses standard cascade of biquad architecture. The second filter uses Cochlea architecture and offers a second order Butterworth response with 500 kHz bandwidth. Of the two filters, the fourth order filter is realized on silicon and fabricated using 0.18 µm standard CMOS technology from United Microelectronics Corporation (UMC). Measurement results reveal that, the filter is power efficient consuming a power of 56.4 µW from 0.5 V supply while offering a dynamic range of 45 dB. The figure of merit (FOM), computed in terms of energy, is found to be 0.355 fJ. When compared with similar low voltage filters realized on silicon, the proposed filter has the lowest FOM. The Cochlea low-pass filter is a proof of concept realization. This filter also uses 0.18 µm n-well standard CMOS process from UMC. Simulation results show that the filter consumes a power of 20 µW operating on 0.5 V supply offering a dynamic range of 51 dB. Simulated FOM is found to be 0.225 fJ. A bias circuit to fix the bulk-transconductance / gate-transconductance of the transistor in the transconductor is proposed based on constant current generating circuit. iiiThis circuit helps to fix the gate/bulk transconductance of a transconductor operating on 0.5 V power supply. A mathematical analysis has been presented in support of the simulation results. The absolute maximum bulk-transconductance deviation from nominal value for the proposed scheme is found to be less than 0.4 % for ±10 % change in supply voltage from nominal 0.5 V, at room temperature and for all process corners. The absolute maximum deviation in transconductance is less than 10 % for the proposed circuit across the process, supply voltage and temperature variations. The two conventional circuits, on the other hand, are found to offer absolute maximum deviation of about 25.8 % and 35.96 %. In the last part of this research, a two-port transmission-line (ABCD) parameter based modeling technique has been presented. This technique accurately models a class of filters in presence of non-idealities of the transconductor such as finite output resistance and parasitic capacitance. In the proposed approach, the filter model is derived/obtained only through CAD simulations as compared with that of conventional state-space method which is based on small-signal equivalent circuit of the filter. The technique has been demonstrated using the second order Cochlea filter.