Figure 10-1 and Figure 10-2 illustrate a printed circuit board (PCB) layout example for the ADS8686S.
- Keep the analog signal away from the digital lines. This layout keeps the analog input and reference signals away from digital noise.
- Use a single common ground plane. For designs requiring split analog and digital ground planes, the analog and digital ground planes must be at the same potential joined together in close proximity to the device.
- Power sources to the ADS8686S must be clean and well-bypassed. As a result of dynamic currents during conversion, each AVDD pin must have a decoupling capacitor to keep the supply voltage stable. Use wide traces or a dedicated analog supply plane to minimize trace inductance and reduce glitches. Use a 10-µF and 0.1-µF ceramic capacitor in close proximity to each analog (AVDD) supply pins (pins 6, 15, 30, and 71) and the digital supply (pin 49).
- Connect the AVDD supply pins (pins 71 and 30) to bypass capacitors on both the top and bottom layers with an isolated via. Use a separate via to connect the bypass capacitors to the AVDD plane.
- Decouple the REFCAP pin (pin 31) with a 10-µF, 0805-size capacitor placed in close proximity to the device pin. Avoid placing vias between the REFCAP pin and the decoupling capacitor.
- Decouple the REGCAP pin (pin 70) with a 10-µF and a 0.1-µF ceramic capacitor placed in close proximity to the device pin. Avoid placing vias between the REGCAP pin and the decoupling capacitor.
- Decouple the REGCAPD pin (pin 52) with a 10-µF and a 0.1-µF ceramic capacitor placed in close proximity to the device pin. Avoid placing vias between the REGCAPD pin and the decoupling capacitor.
- If the internal reference of the device is used, decouple the REFIO pin to REFIO_GND with a 10-µF, X7R-grade, 0603-size ceramic capacitor. Place the capacitor on the top layer, close to the device pin. Avoid placing vias between the REFIO pin and the decoupling capacitor.
- Connect all ground pins (AGND) to the ground plane using short, low-impedance paths and independent vias to the ground plane.