Switched-Capacitor Filter Cutoff

Formula

Description

Switched-capacitor filters use a clock signal to sample charge packets on capacitors, effectively implementing resistors whose value is R_eq = 1/(C × fclk). The filter cutoff frequency is set by the clock frequency divided by a fixed ratio determined by the IC design (commonly 50:1 or 100:1). This allows precise frequency tuning by simply changing the clock frequency, without changing any component values. Switched-capacitor filters are used in anti-aliasing, data acquisition, and telecommunications where tunable or precise filter frequencies are needed.

Variables

• fc — Filter cutoff frequency (Hz)
• f_clk — Clock frequency (Hz)
• ratio — Clock-to-cutoff frequency ratio (e.g., 50, 100)

Practical Notes

Common clock-to-cutoff ratios: MAX291/MAX292 use 100:1, LTC1062 uses 100:1, MF10 uses 50:1 or 100:1 (selectable). The clock frequency must be much higher than the signal frequency to avoid clock feedthrough artifacts. An external RC filter after the switched-cap filter removes clock feedthrough. Accuracy of the cutoff frequency depends on clock accuracy, typically ±0.2% with a crystal oscillator.