DPPs program key parameters of bandpass filter
Edited by Bill Travis
By Chuck Wojslaw, Catalyst Semiconductor, Sunnyvale, CA -- EDN, December 12, 2002
The three-amplifier implementation of the state-variable filter in Figure 1 provides for second-order bandpass, highpass, and lowpass responses. The strength of the circuit, however, is in the bandpass response (VOUT/VIN), in which it's easy to achieve high gain (G) and high Q. These two characteristics are important in applications in which selectivity is a key parameter in the filter. The application value of the circuit becomes even greater when DPPs (digitally programmable potentiometers) control and vary the bandpass filter's center frequency, f0, and passband gain, G. The independent control of a bandpass filter's parameters is a rarity among traditional filter-circuit techniques.
For the filter, IC1 functions as a summing amplifier, and IC2 and IC3 function as integrators. Three potentiometers, DPP1, DPP2, and DPP3, program the center frequency and passband gain of the filter. The 100-tap Catalyst potentiometers, DPP1 and DPP2, are electronically ganged to program the frequency, f0, and DPP3 programs the gain, G. The center frequency is f0=1/(2πR5C1)=1/(2πpRC1), and the gain is G=R2/R1=(1–p)/p, where p represents the relative position of the wiper from one end (0) of the potentiometer to the other end (1). R represents the DPP's end-to-end resistance. The gain and Q are related by Q=(1+G)/2. The increment/decrement interface of the CAT5113 DPP allows for real-time, closed-loop applications, in which you can continuously vary the center frequency of the filter to track an input signal or to accommodate a system-level requirement. For the values shown, the center frequency varies from less than 200 Hz to more than 6 kHz with gain values as high as 25.
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