Design of Low-Voltage Highly Linear Switched-R-MOSFET-C Filters

This paper discusses the design, analysis and performance of a low-voltage, highly linear switched-R-MOSFET-C filter. High linearity, even at a low supply voltage, is achieved through the use of duty-cycle-controlled tuning. Tuning MOSFETs are switched completely on while conducting, such that their nonlinear resistance is much smaller than the linear filter resistors, resulting in low distortion. The MOSFETs are also placed inside the filter feedback loop which further reduces distortion. Because tuning is done in the time domain, rather than in the voltage domain, the tuning range is independent of the supply voltage. The filter achieves -77 dB total harmonic distortion (THD) using a 0.6-V supply, and -90 dB THD using a 0.8-V supply, with a 0.6-Vpp differential 2 kHz sine input. The prototype IC, implemented in a 0.18-mum CMOS process, occupies an area of 0.7 mm² and consumes 1 mW of power from a 0.6-V supply.