I am currently troubleshooting audio noise susceptibility on a PCB using the PCM3794 Audio CODEC. This has brought up some questions concerning the grounding of the chip that are not answered in the data sheet. There is more than one ground pin on the chip, and I would like to avoid any unnecessary ground loops to minimize the noise pickup.
How should PGND, AGND, DGND and thermal pad be connected on PCB? The current configuration is DGND and thermal pad connected to the digital ground plane while PGND and AGND are connected to the analog ground plane. The single interconnection point between the analog and digital ground is located quite a distance from the CODEC chip. Should this connection point ideally be moved as close to the CODEC chip as possible?
During
my test process, I measured the resistance between the pins on a chip that was not soldered to the board and got the following:
AGND to DGND - 16.5 ohms
AGND to PGND - 25.4 ohms
DGND to PGND - 27.2 ohms
Thermal Pad - not connected to any of the GND pins (AGND, DGND, PGND)
There is not much information about the ground pins in the data sheet, no connections shown on the functional diagram, but they seem to have a significant relation to one another.
Does anyone have some insight in the applications of the PCM3794 or similar CODEC? It would be good to find out what the intended uses of the separate ground pins are.