For ACAPs without dedicated sense pins, the sense line connections should connect to BGA balls that are as close as possible to the most significant point-of-load on the ACAP. This can be best estimated via DC simulation or via the device view in the Vivado tools. If the point-of-load cannot be estimated, placing the sense connections towards the center of the BGA pin field is recommended. Placing at the ball furthest from the regulator is also an option, though less preferred.
- Place inline 0Ω resistors close to the ACAP:
- Resistors are required to ensure sense lines are routed as traces and not dropped to the plane.
- Route sense lines as 50Ω loosely coupled differential transmission
- Ground is required to sense true differential between power and ground.
For optimum power transfer, ensure that the balls connecting to the sense lines fully connect to their respective planes, i.e., do not leave them unconnected as “spy holes” (see the following figure).