Stable, digitally programmable active filters

The incorporation of digitally controlled gain linear amplifiers in the loops of a state variable active filter is shown to result in a filter digitally programmable in frequency and selectivity. The filter is shown to exhibit an order of magnitude higher temperature stability than programmable filters employing analog multipliers. Experimental results verifying the high temperature stability are presented for a filter programmable with 8 b over the frequency range 100 Hz to 25.5 kHz.