12.1 Layout Guidelines

Employ best design practices when laying out a printed circuit board (PCB) for both analog and digital components. The PCM186x-Q1 audio ADC is a relatively simple device to lay out, even on a two-layer PCB. The following basic recommendations for layout of the PCM186x-Q1 help achieve the best possible performance of the device.

• Separate analog and digital sections where layout permits. Route analog lines away from digital lines. This routing technique prevents digital noise from coupling back into analog signals.
• The bottom copper plane can be a shared ground, whereas a ground plane can be used on the top layer as well. Separated planes for analog and digital grounds are not required to achieve data sheet performance.
• Place decoupling capacitors as close as possible to the supply pins, and in the same layer of the device, to yield the best results. Do not place vias between decoupling capacitors and the device.
• Place ground planes between the input traces to achieve the lowest crosstalk performance.

The EVM user’s guide shows the schematics, a bill of material, and a more detailed PCB layout.

12.1.1 Grounding and System Partitioning

Use the same plane for analog and digital grounds to avoid any potential voltage difference between these grounds. On the PCM186x-Q1 EVM, maximum SNR performance is achieved by using a single ground plane, and making sure that the return currents for digital signals are not near the AGND pin or the input signals.

As shown in Figure 74, the pin layout of the PCM186x-Q1 is partitioned into two sections: analog and digital. No digital return currents (for example, clocks) are generated in the analog circuitry, as long as the system is partitioned in such a way that digital signals are routed away from the analog sections.

Figure 74. Single Ground With Analog Pins Partitioned to the Top and Digital Pins at the Bottom

12.2 Layout Example

Figure 75. Layout Example