A charge-pump-controlled MOSFET-C single-amplifier biquad

The spurious-free dynamic range (SFDR) of a MOSFET-C filter can be increased greatly by generating its tuning voltage with a charge pump. In this paper, we apply this technique to build a Sallen-and-Key lowpass filter with a pole frequency of 24 MHz and a pole Q of 3. It has an SFDR better than 50 dB and consumes 16 mW from a 3.3 V supply. Implemented with a double-poly triple-metal 0.6-/spl mu/m CMOS process, it covers an area of only 0.11 mm/sup 2/. In addition to a description of the filter and the charge pump, we also discuss linear and non-linear clock feed-through from the charge pump's own ring oscillator, and derive a formula for the optimum voltage swing at the MOSFET-C network nodes.

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