Passive switched-capacitor filter with complex poles for high-speed applications

A novel complex-pole switched-capacitor lowpass filter topology is presented. The proposed scheme features sharper roll-off in the frequency response and reduced in-band loss compared with a real-pole filter. It provides higher stop-band rejection for the same bandwidth. Hence, the required filter order and chip area are reduced. These improvements are achieved without adding more active blocks to the real-pole filter, and with minimal added complexity. The proposed filter has the same low-output thermal noise level as the real-pole filter. These features, combined with a wide-frequency tuning range, make the filter suitable for high-speed, low-noise, and low-power applications. A fourth-order 200 MS/s filter prototype was designed in 0.18 µm Asahi Kasei Microdevices Corporation CMOS technology. Simulations verify that the average in-band loss is reduced by 3.2 dB compared with the real-pole filter. The 3 dB cut-off frequency can be tuned from 470 kHz to 17.5 MHz with 100 dB maximum stop-band rejection.