TPS8804EVM: Question concerning TPS8804EVM AGND pin and its layout

Zt Liu

Prodigy 140 points

Part Number: TPS8804EVM

Dear TI team,

Just got the lovely TPS8804EVM.

Before doing the experiments, I have some question about the AGND pin and its layout .

1. Is AGND (pin 5) already internally connected to the DGND pin (pin32) and/or thermal pad and/or PGND pin (pin 17)?

2. Why in the layout of TPS8804EVM, AGND pin externally connects to nothing but only an isolated place. (see belowed)

Why can't this pin connect externally to the thermal pad? Or do I make any mistake?

3. In the Datasheet, AGND (pin 5) is described as "Analog ground. Connect to ground plane." Does this statement contradict to the layout of EVM board?

(I've measured that there is no significant resistance between this isolated AGND plane and other ground plane.)

(Top side of the board)

(Bottom side of the board)

Looks like that at least the AGND Pin is internally connected to the thermal pad.

And I am not sure about why the pin 4 is left in an isolated plane externally.

Thanks for reading this post, and hope someone will answer my question.

Zt.

over 3 years ago

0 Aaron Goodson over 3 years ago

TI__Expert 6330 points

Hi,

I have notified our expert regarding this topic. Please expect a response by 4/29/20.

Thanks,

Aaron

0 Grant Falkenburg over 3 years ago

TI__Intellectual 1965 points

Hi Zt.,

Thanks for your question.

The AGND, DGND, PGND, and thermal pad are all isolated. The connection from all of these to a central ground reference needs to be done on the PCB.
This assessment is accurate given the information you have. However, the PCB is 4-layer and an internal layer connects AGND to the GND plane and thermal pad. It is good to connect AGND directly to the thermal pad similar to the DGND connection.
It is connected on the EVM (see #2).

I understand how these layout diagrams can be confusing given that two of them are missing. I will look into updating the user guide with the internal layers.

Let me know if you have any other questions.

Best regards,

Grant

0 Zt Liu over 3 years ago in reply to Grant Falkenburg

Prodigy 140 points

Thank you so much, Grant!

I didn't know that it's a four layer board.

That really answered my question.

I am wondering if we can have the design file of TPS8804EVM, or at least the gerber files?

In case we have more questions like this.

Best regards,

0 Grant Falkenburg over 3 years ago in reply to Zt Liu

TI__Intellectual 1965 points

Hi Zt,

Here are some images of the inner layers. I can send you the Gerber files if needed but it will take a few days to get the files ready.

Best regards,

Grant

0 Zt Liu over 3 years ago in reply to Grant Falkenburg

Prodigy 140 points

Thank you so so much, Grant!

Please send me gerber files, there is so much we can learn from your design!

zhihtailiu@gmail.com

Best Regards,

Zt.

0 Grant Falkenburg over 3 years ago in reply to Zt Liu

TI__Intellectual 1965 points

Hi Zt,

No problem at all. I will prepare the files and will let you know when they're ready. Please be patient as it can take a few days.

Best regards,

Grant

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TPS8804EVM: Question concerning TPS8804EVM AGND pin and its layout