DPPs program key parameters of bandpass filter

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 (V_OUT/V_IN), 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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