4.4 Use Case 4 Output Filter Computation

For Use Case 4: Stereo Single-Ended Audio DAC with Active Low-Pass Filter, the circuit described in the device data sheets uses a single operational amplifier. It can only generate one double (complex) pole and two single poles, and therefore cannot match the usual fourth-order filters (such as Butterworth, Bessel or Linkwitz-Riley). That is why it was optimized as a third-order Butterworth filter to obtain the flattest response, and the remaining single pole was moved upward in the frequency band to avoid interference. It also targets unity gain in the bandwidth.

A single operational amplifier active filter design tool (Excel spreadsheet) is available in the MPLAB® Mindi™ Analog Simulator Software Library (see the Appendix). Use this tool to compute passive components for filters up to the fourth order, employing some design tricks to overcome the limitations pointed out above. As the component count depends directly on the filter order, it is up to the designer to determine the best compromise for a given application. Furthermore, an order higher than the second may require some tuning iterations to precisely match a given response.