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Power Supplies and Grounding

D/A Output Filter Circuits

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PCB LAYOUT GUIDELINES

PCM1780, PCM1781, PCM1782

SLES132A – MARCH 2005 – REVISED APRIL 2005

APPLICATION INFORMATION (continued)

Proper power supply bypassing is shown in Figure 28. The 0.1-

µF ceramic capacitor and the 10-µF electrolytic

capacitor are recommended.

Delta-sigma D/A converters use noise-shaping techniques to improve in-band signal-to-noise ratio (SNR)

out-of-band noise must be low-pass filtered in order to provide optimal converter performance. This is

accomplished by a combination of on-chip and external low-pass filtering.

Figure 27(a) and Figure 29 show the recommended external low-pass active filter circuits for single- and

dual-supply applications. These circuits are second-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 Dynamic Performance Testing of Digital Audio D/A

Converters (SBAA055), available from the TI Web site at http://www.ti.com.

Figure 29. Dual-Supply Filter Circuit

A typical PCB floor plan for the PCM1780/81/82 is shown in Figure 30. A ground plane is recommended, with the

analog and digital sections being isolated from one another using a split or cut in the circuit board. The

PCM1780/81/82 should be oriented with the digital I/O pins facing the ground plane split/cut to allow for short,

direct connections to the digital audio interface and control signals originating from the digital section of the

board.

Separate power supplies are recommended for the digital and analog sections of the board. This prevents the

switching noise present on the digital supply from contaminating the analog power supply and degrading the

dynamic performance of the PCM1780/81/82. In cases where a common 5-V supply must be used for the analog

and digital sections, an inductance (RF choke, ferrite bead) should be placed between the analog and digital 5-V

supply connections to avoid coupling of the digital switching noise into the analog circuitry. Figure 31 shows the

recommended approach for single-supply applications.

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