Add Digital Adjustment to Continuous-Time Filters

Abstract: This design note shows how adding digital-to-analog converters (DACs) provides digital tuning to continuous-time filters. A dual filter, dual op amp, and dual DAC form an adjustable bandpass circuit with a constant 500Hz bandpass over the 5kHz to 20kHz center-frequency range. The MAX275 continuous-time filter and MAX412 op amp are featured in this design.