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Author Topic: One Board, Two Grounds  (Read 802 times)
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Is it useful to create two different grounds for two different power supplies on a single circuit board (EAGLE CAD)? Honeywell's HMC5883L datasheet has a reference schematic for using two different power supplies (pg. 6) - say, VDD at 3.3V and VDDIO at 1.8V. Do we need to have two different Ground planes for these two devices? Does it relate to having different analog and digital grounds?


* Screen Shot 2012-12-31 at 1.37.22 PM.png (98.05 KB, 811x540 - viewed 29 times.)
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SE USA
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you want your grounds connected when using multiple DC sources
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Thank you Osgeld for clarification. I was confused after seeing this post about analog/ digital ground planes.
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Thank you Osgeld for clarification. I was confused after seeing this post about analog/ digital ground planes.


It can be confusing but on boards with mixed low level analog circuitry and digital switching circuitry there is advantages to laying out separate ground paths/planes/traces to keep the current flowing in the two ground sections separate from each other, but there will always have to be an electrical common connection made between these two ground paths somewhere. Likewise it good to have high current flow paths for say motors run on separate traces to keep more sensitive components somewhat isolated for the high ground current path, but again the grounds will still have to be electrically connected to each other somewhere, usually in a preferred single 'star grounding' point.

Grounding and circuit common can be a very complex subject when dealing with noise minimization and it's all about proper PCB layout, ground planes and trace routings.

Lefty
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India
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Thanks Lefty. That pretty much clears up everything.
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the land of sun+snow
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Thank you Osgeld for clarification. I was confused after seeing this post about analog/ digital ground planes.

Hmm, guess I blew it again, huh.
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As a follow up to the question of separate grounds, I have a design that also calls for two separate 5v supplies.  What's the best way to do this? Two different but identical regulators?
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Manchester (England England)
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I have a design that also calls for two separate 5v supplies.
You have to ask yourself why.
Normally a good bit of decoupling will be enough for you to use the same supply.
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I have a design that also calls for two separate 5v supplies.
You have to ask yourself why.
Normally a good bit of decoupling will be enough for you to use the same supply.

The data sheet for my external ADC says the best design scheme is to power AVdd and DVdd with two separate supplies.
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 and use chokes to connect Agnd ground planes and Dgnd ground planes together, about 100 uHy and capacitors to the respective grounds for either ground plane. 3  capacitors are typically required a 10 nF, a 100 nF and a 1 uF are the typically recommended values. The reason was stated by Thevenin, the chokes just keep the noise where it belongs. This method though is typically applied to circuitry that works with very low level measurements and not always required. The basic requirement is to connect the ground planes together where the main filter capacitor is grounded thus forcing a 'required' star topology for power and ground.

Bob
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The data sheet for my external ADC says the best design scheme is to power AVdd and DVdd with two separate supplies.
That is data sheets for you. It does not mean you have to have two supplies, it is just what they tested it at to minimise things like ripple, cross talk and the like.
So rather than design it correctly they just show the minimum of what you need. This is not a recommendation to use two supplies but a bit of a warning that they need to be decoupled properly.
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