Connecting AGND and GND on different layers

Hi everyone,

I want to use the Texas Instruments ADS7953 ADC in a design. But I am struggling to connect AGND and GND on the PCB.
In the ADS7953 datasheet it says:
“A copper fill area underneath the device ties the AGND, BDGND, AINM, and REFM pins together. This
copper fill area must also be connected to the analog ground plane of the PCB using at least four vias.”

The IC is mounted on the bottom layer, and has a AGND copper pour underneath it. On the bottom layer, there’s a GND pour. How do I connect the two layers? I’d like to use a net tie if possible, directly connecting AGND and GND in the schematic should be the last solution.

Best regards,

Spatz

Hello, welcome to the forum!

There are several parts to this answer.

  1. In the schematic you need to add AGND and GND symbols. Connect them with a “NET-TIE-2” (or whichever symbol you prefer. Create a new Netlist and Save it.
  2. Assign a Footprint to Net-Tie
  3. In PCBNew, on the bottom layer you have AGND. Create a small AGND Zone on the TOP layer, under the IC. Add multiple small VIAs between top and bottom AGND Zones.
  4. On the top layer, place the Net-Tie Footprint, linking the 2 zones together.

Personally, I would add a ferrite bead symbol and footprint in parallel with the Net-Tie. Later you can either use the bead or the shorted Net-Tie (you would have to cut a small trace to enable the bead).

Also, I would do everything else with the layout and make the Net-Tie the very last step of the design. There can be problems with Gnd/AGND being tied in the netlist. So do everything else first.

I hope this helps

If I would expect that may be I will have to use ferrite bead I would place only 0R 0603 symbol there. Assembling extra 0R (when decided to connect directly) seems being much more simple (in production) then cutting the track if decided to use ferrite bead (0603).

Yes 0R or Bead is acceptable. Be sure to include that in the schematic.
Actually, it may be better this way as the Netlist will not consider the Gnd and Agnd are tied together and doing the PCB layout will be easier.

Good luck!

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