Hello,
I was hoping to get some confirmations on my finding regarding the TPD6E004 ESD Protection Diode. We have this part hooked up to our USB 2.0 interface. The part is located near the connector where the USB data lines come in. We have tied the GND pin of TPD6E004 to our AGND, which is also our chassis. From what I've read, it is recommended to tie the GND pin to the ground plane associated with the chassis. The USB data lines go into our digital side of our PCB, which means their return/reference point is DGND. However, we have noticed some issues with our USB Interface when we turn off the analog side and turn it back on. When turning off the analog side, the USB interface still works, however, when we turn on the analog side on, the USB connection drops. We measured about 300-400 mV of AC noise btw DGND and AGND when the analog side is turned back on. There is no AGND reference plane underneath the TPD6E004 part in the layout; it connects straight to the surrounding copper planes on the board edges that will touch with the chassis.
Is it safe to assume that the noise from AGND is allowing energy to get to the USB data lines when the analog side is being turned on again, thus causing issues with the USB connection dropping? We changed the connection of the GND pin from AGND to DGND and have not seen the USB interface drop after powering on the analog side again. Any input is greatly appreciated and any pointers to TI articles about correctly referencing the GND pin of TI's ESD parts would also be appreciated.
