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Powering AVDD Separately from DVDD

 
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jakesteffes



Joined: 09 May 2013
Posts: 5
Location: United States

PostPosted: Fri Sep 16, 2016 10:54 am    Post subject: Powering AVDD Separately from DVDD Reply with quote

Hi,

In the interest of reducing noise, is it of any benefit to use a separate regulator for AVDD (isolating it from DVDD's regulator)? If so, should REFP be referenced to the same source as AVDD, or should it be referenced to DVDD?

Additionally, is there a specific 3V3 regulator that should be used? I've been using a run-of-the-mill 78L33, and I'm curious if finding a lower noise regulator would be beneficial.

Thank you!
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frank



Joined: 19 Oct 2005
Posts: 972

PostPosted: Fri Sep 16, 2016 2:19 pm    Post subject: Reply with quote

There are many things that can contribute to noise so it may or may not have any benefit. Careful layout can be the best way to remove noise.

If you do use separate supplies then REFP should reference AVDD.

Quality regulators always help but good supply filtering, bypass caps close to the pins of ICs, etc. all play a role. Avoid regulars from bargain sources, no guarantee they are real and even if real they could be rejects that are being passed off as good.
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Frank Thomson
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jakesteffes



Joined: 09 May 2013
Posts: 5
Location: United States

PostPosted: Fri Sep 16, 2016 4:09 pm    Post subject: Reply with quote

Helpful information - thanks Frank!
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Digital Larry



Joined: 12 Nov 2012
Posts: 274

PostPosted: Sat Apr 08, 2017 7:33 am    Post subject: Reply with quote

Spec sheet shows (analog + digital) current <= 70 mA. Supposing I did want to power them separately, is there a spec for max current for each section?
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frank



Joined: 19 Oct 2005
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PostPosted: Sun Apr 09, 2017 9:00 am    Post subject: Reply with quote

Uh, no. I don't think we ever measured them separately, may need to try to get time to do that.
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Digital Larry



Joined: 12 Nov 2012
Posts: 274

PostPosted: Mon Apr 10, 2017 10:43 am    Post subject: Reply with quote

HI Frank,

Moving this conversation forward, I am looking at some places that do 4-layer protos and at least one of them only allows internal planes with either isolated or connected vias. So, even though I can imagine a "split" power plane under the chip, it is not possible with this vendor.

If one were to use separate supplies for AVDD and DVDD then I presume that the digital signals would benefit from being supplied by a plane more so than analog, along with all of your other points. This would leave AVDD to find its way on the top or bottom "signal" layer but that's not a huge deal. I just want to get your take on whether a separate regulator for AVDD is going to buy much by the time I've gone 4-layer and done everything else correctly.

Couple more questions, assuming one does use separate AVDD:

a) Is there any benefit to RC low pass filtering AVDD before it gets to the VREFP pin? I'm looking at a schematic that does this, but don't know what the internal impedance of that pin is, so am not sure what the RC values want to be.

b) Should the control pots be referenced to AVDD or DVDD?

Thanks,

DL
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frank



Joined: 19 Oct 2005
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PostPosted: Mon Apr 10, 2017 1:46 pm    Post subject: Reply with quote

Good questions. If you have 4 layers and good ground then there is little to gain from split Vdd.

As to LP filtering the AVDD line if splitting Vdd I have found that to help only when using a switching regulator that operates close to the audio range. I prefer to avoid that if possible and use a linear regulator.

I am not sure if the POTs use AVDD or DVDD internally but I would use DVDD in a split case. Will try to get time to look at the layout and see.

Don't understand why you can't have a split power plane under the chip, it is just 2 pours one to DVDD and one to AVDD and the chip straddles across them. Of course that works best if the pours are both large and power other devices not if the AVDD pour would be an island in the middle of the DVDD.

And a tip for everyone working on a PCB, make sure your ground plane extends past all other copper around the edge of the board.
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