Switch capacitor filter design aids

Abstract

Two aids for the design of switched-capacitor filters (SCFs) are examined: breadboard modelling and computer simulations. A breadbroad was constructed based upon the same passive prototype of a seventh-order integrated filter. The breadboard met the original design specifications better than the integrated filter because the schematic and parasitics of the integrated filter were unknown.

Three computer programs, SPICE, DIANA, and TCAPS, are compared to determine their abilities and limitations in SCF design. SPICE is of limited value since it models switched capacitors as resistors. DIANA and TCAPS directly simulate the digital nature of switched capacitors, so they track in their predictions of

1) increased passband ripple,

2) increased cutoff frequency,

3) increased third notch frequency,

4) increased OBR.

On the basis of this comparison, it is not possible to determine if DIANA or TCAPS is the better program. However, both DIANA and TCAPS simulate SCFs far more accurately than SPICE.