11.1 Power Supply, Grounding, and Bypassing

In a mixed-signal system design, the power-supply and grounding design plays a significant role. The device distinguishes between two different grounds: AVSS (analog ground) and DVSS (digital ground). In most cases, laying out the PCB to use a single ground plane is adequate. However, in high-frequency or high-performance systems, care must be taken so that this ground plane is properly partitioned between various sections within the system to minimize interactions between analog and digital circuitry. Alternatively, the digital supply set consisting of the DVDD_1P8, DVDD_1P2, and DVSS pins can be placed on separate power and ground planes. For this configuration, tie the AVSS and DVSS grounds together at the power connector in a star layout. In addition, optical or digital isolators (such as the ISO7240) can completely separate the analog portion from the digital portion. Consequently, such isolators prevent digital noise from contaminating the analog portion. Table 45 lists the related circuit blocks for each power supply.

Reference all bypassing and power supplies for the device to their corresponding ground planes. Bypass all supply pins with 0.1-μF ceramic chip capacitors (size 0603 or smaller). In order to minimize the lead and trace inductance, the capacitors must be located as close to the supply pins as possible. Where double-sided component mounting is allowed, these capacitors are best placed directly under the package. In addition, larger bipolar decoupling capacitors (2.2 µF to 10 μF, effective at lower frequencies) can also be used on the main supply pins. These components can be placed on the PCB in close proximity (< 0.5 inch or 12.7 mm) to the device itself.

Bypass the VCM pin with at least a 1-μF capacitor; higher value capacitors can be used for better low-frequency noise suppression. For best results, choose low-inductance ceramic chip capacitors (size 0402, > 1 μF) placed as close as possible to the device pin.