Delta-sigma D/A converters use noise-shaping techniques to improve in-band signal-to-noise ratio (SNR) performance at the expense of generating increased out-of-band noise above the Nyquist frequency, or fS/2. The out-of-band noise must be low-pass filtered in order to provide the optimal converter performance. This is accomplished by a combination of on-chip and external low-pass filtering.
Figure 25(a) and Figure 31 show the recommended external low-pass active filter circuits for single- and dual-supply applications. These circuits are 2nd-order Butterworth filters using the multiple feedback (MFB) circuit arrangement, which reduces sensitivity to passive component variations over frequency and temperature. For more information regarding MFB active filter design, see Burr-Brown applications bulletin (SBAA055), available from the TI Web site at http://www.ti.com.
Because the overall system performance is defined by the quality of the D/A converters and their associated analog output circuitry, high-quality audio operational amplifiers are recommended for the active filters. TI's OPA2353 and OPA2134 dual operational amplifiers are shown in Figure 25(a) and Figure 31, and are recommended for use with the PCM1754 device.