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ADAU1701 Analog Ground Plane

Category: Hardware

I'm designing a 4-layer board with the ADAU1701.

On the board is also:

Buck regulator for ADAU1701 digital power

Linear regulator for ADAU1701 analog power

Linear regulator for op-amps

Op-amps for input and output filters

Will I attain best performance from the ADAU1701 if I use a single ground plane for all ADAU1701 components as the datasheet suggests? Or will it work better with a separate analog ground plane (joined to digital under the IC) for AVDD, PVDD, CM, PLL-LF, FiltA, FiltD components?

Additionally, would you recommend a ground island around the crystal and load capacitors?


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[edited by: ssashton at 3:42 PM (GMT -4) on 3 May 2022]
  • My experience/general rule for pretty much any non-RF design (and probably applies to many RF designs too, but I don't do RF) is one solid ground plane for everything.  A design that doesn't do this might work but you'll probably never get through EMI testing.

    The 1701 parts analog performance isn't that high that you need to be all that worried.  By high I mean > 115 dB or so SNR. Don't make dumb mistakes and you'll be fine.  The trick of course is knowing which ones are the dumb ones vs. the non-dumb ones.

    I'll close with the well known joke among EMI consultants (I am not one, but been around enough of them):

    Q: What do you call a person that splits their ground planes?

    A: A customer

  • ADAU1701 work fine with a single ground plane for all analog and digital. The datasheet does not encourage splitting ground planes. Splitting ground planes is not encouraged for good EMC.

    Use all the switching regulators you please to, form some good R&D by comparing them with linear regulators if you must...

    Seek quality ADCs with respectable dynamic range if the performance of the internal one is any trouble,
    noise gates can help a lot if this bothers you in your application (every commercial application I've encountered uses them), this forum has some good examples.

  • I did test MP2359 switching reg against LM317 on the analog 5V supply. The noise floor had fewer spurious products with the LM317.

    I know MP2359 is not a high performance switching reg but it tells me there is benefit to using linear regs on the analog supply.

    I did produce a 4 layer solid ground plane board. Prototype in production now, we can see how it performs compared to my older 2 layer grounded by star point and traces with separate star for analog and digital. If I remeber I'll report back here.

  • A linear regulator supply is always quieter, and really should be used all the time unless it's burning too much power.  (i.e. from 5V to 3.3V should be linear, probably not 12V to 3.3V).

    A proper 4-layer board is so much better than a 2 layer, and these days doesn't cost much more.  Very much worth it for time savings alone.  A good solid ground plane is best practice.

  • Hello Joe42,

    I agree with your basic advice. We ( ADI) have plenty of new buck regulators that are really quiet and do not require much for external components. They switch at around 1-2MHz so WAY out of the audio range so they can be a good option when someone needs to come down from 12-24V or more down to 3.3V. 

    Consult with our power group for more details. 

    Dave T

  • Good to know!  Being a little old-school my preference is still to drop to 5V via switching, then 3.3V by linear, but with the new parts available it's probably no longer necessary.  (And usually I need "dirty" 5V elsewhere anyway..)