Embedded anti-aliasing in switched-capacitor ladder filters with variable gain and offset compensation

A combination of continuous-time and switched capacitor integrators in a simulated LC lossless ladder yields a response with suppressed aliasing without the use of continuous-time prefiltering. Fabricated in a 0.35-μm CMOS process, a fifth-order Cauer low-pass filter for a W-CDMA cellular phone receiver has a cutoff frequency of 1.92 MHz and aliasing suppression of better than 40 dB for 30.72-MHz sampling. Without using any tuning mechanism, a 10% accuracy of the cutoff frequency is achieved. As additional features, the filter has variable gain from -13.3 to 16.4 dB and an offset compensation mechanism. With the latter, a 50-mV dc offset added to the input is suppressed to 11 mV or less at the filter output under the maximum gain setting. The filter consumes 2.81 mA at 1.8-V power supply in a die area of 0.62 mm²