This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS8557: Digital and Analog GND connection under the device when using multiple ADCs

Genius 15700 points
Part Number: ADS8557

Hello,

My customer is designing their product with multiple ADS8557s.
There is a layout recommendation in Figure 47 of The ADS8557 datasheet(SBAS404D).
From this recommendation, digital GND and analog DNG and connected at one point under the device.
I think this is relatively common recommendation for ADCs.
The question is when using multiple ADCs on the board, analog and digital GND should be connected at one point under all the ADCs?

Regards,
Oba

 

  • Hi Oba,

    Using a single common ground plane is strongly recommended. If the customers do not split the ground well (actually this is common), even short undershoots on the digital interface lower than –300 mV lead to the conduction of ESD diodes causing current flow through the substrate and degrading the analog performance. Hence, a single solid ground plane for the entire printed circuit board (PCB) or a dedicated analog ground area is recommended.This is especially important when designing a circuit with multiple ADCs on the same circuit board.

    Best regards,

    Dale

  • Hello Dale,

    Thanks for your reply.
    I want to confirm what we should do for the case that multiple ADCs are on the same board.
    Which is better from the below figure?

    Regards,
    Oba

  • Hi Oba,

    Both of them are not good design. As I said, using a single common/solid ground plane is strongly recommended.

    Best regards,

    Dale

  • Hello Dale,

    Thanks for your reply.
    Let me confirm you meaning because I'm a little bit confused.
    The ADS8557 datasheet recommends the below layout which analog GND and digital GND are separated and both are connected at one point under the ADC.
    But you recommend to use a single GND plane instead of separating analog and digital GND.
    Do you recommend using a single GND plane for only using multiple ADCs case? Or Do you also recommend it to single ADC case?

    Regards,
    Oba

  • Hi Oba,

     A single common ground plane is commonly recommended for the design using multiple ADCs. For a single ADC design, the customers can split the ground if they can split it well. However, if the customers do not split the ground well (actually this is common for many engineers), even short undershoots on the digital interface lower than –300 mV lead to the conduction of ESD diodes causing current flow through the substrate and degrading the analog performance. This is the reason why a single common ground plane is always recommended in new ADCs released during past years whether it is one or multiple ADCs, for example, ADS8588S, please see the layout guidelines in page 52. Also, we always used a single common ground plane for the EVM designs, see ADS8588SEVM as an example. I hope my explanation is helpful for you to understand.

    Best regards,

    Dale

  • Hello Dale,

    Thank you very much for your reply.
    I understand well.

    Regards,
    Oba