A 8.25-MHz 7th-order Bessel filter built with MOSFET–C single-amplifier biquads

The 7th-order Bessel filter presented in this paper has an edge frequency that is continuously tunable from 4.5 MHz up to 10 MHz. It was fabricated with the 0.6-micron CMOS process by AMS, covers a chip area of 0.24 mm2, and consumes 49 mW from a 3.3-V supply. The SNR at −40 dB of harmonic distortion is between 48 dB and 50 dB over the whole tuning range. The comparatively low power consumption and chip area could be achieved by using single-amplifier biquadratic building blocks implemented as MOSFET–C filters and generating the control voltage of the MOSFET resistors with an on-chip charge pump. This paper briefly discusses design issues of such filters, sources of harmonic distortion, clock feed-through, and noise. Finally, it is shown that the filter can also be implemented using MOSFET capacitors instead of poly-poly capacitors with only 4 dB loss of SNR.