12.1 Layout Guidelines

Good layout practices are crucial to realize the full performance of the ADC. Poor grounding can quickly degrade the noise performance. This section discusses layout recommendations that help provide the best results.

For best performance, dedicate an entire PCB layer to a ground plane and do not route any other signal traces on this layer. However, depending on layout restrictions, a dedicated ground plane may not be practical. If ground plane separation is necessary, make a single, direct connection to the planes at the ADC. Do not connect individual ground planes at multiple locations because this configuration creates ground loops.

Route digital traces away from the CAPP and CAPN pins, away from the REFOUT pin, and away from all analog inputs and associated components in order to minimize interference.

Because large capacitance on DOUT/DRDY can lead to increased ADC noise levels, minimize the length of the PCB trace. Use a series resistor or a buffer if long traces are used.

The internal reference output return is the AGND pin. To minimize coupling between the power supply and reference-return trace, route the traces separately; ideally, as a star connection to the AGND pin.

Use C0G capacitors on the analog inputs and for the CAPP to CAPN capacitor. Use ceramic capacitors (for example, X7R grade) for the power-supply decoupling capacitors. High-K capacitors (Y5V) are not recommended. The REFOUT pin requires a 10-µF capacitor and can be either ceramic or tantalum type. Place the required capacitors as close as possible to the device pins using short, direct traces. For optimum performance, use low-impedance connections with multiple vias on the ground-side connections of the bypass capacitors.

When applying an external clock, be sure the clock is free of overshoot and glitches. A source-termination resistor placed at the clock buffer often helps reduce overshoot. Glitches present on the clock input can lead to noisy conversion data.