Dan:
Figure on a standard opamp model with unity gain-bandwidth of 5 MHz (at high power) and output resistance of about 20 ohms. The standard multiple loop RC-active topology has center frequency that is sensitive to GBW by Q^2. There's another topology in Moschytz and Horn for "medium Q" with feedback to the non-inverting input that is sensitive by only 2*Q (that's bad but better) . For the first case, a typical filter at 170 kHz and Q=5 gets pushed down to 125 kHz and is VERY sensitive to GBW, the second topology gets pushed down to 155 kHz, is less sensitive, but has two more parts so that much more accumulated tolerance error. This is a case where lots of GBW doesn't help. If you design counting on GBW=5 MHz, and we deliver at GBW = 8 MHz, then your design shifts a bunch. As a rough rule, look for GBW = Gain(inband) * Fcenter * Q * 30 for this general topology. PSoC 3 is inadequate for this application by a factor of about 5. If you need better, consider the switched capacitor filters in PSoC1, good to 200 kHz but noisy, or using an external opamp with a whole lot of GBW or trusty inductors and capacitors.
---- Dennis
(seg@cypress.com)
