[time-nuts] Two V(in)s walk into a bar...

[Bob Camp]

Hi

The real answer is a bit complicated. (As in at least a week of class, I sitting in a refresher course now),

Ground = no current in normal use
Return = where the IC ground pin connects

A lot of confusion goes on between return and ground. In about 99.99% of schematics return is miss-labeled as ground.

In your case, you need to be careful about layout. Keeping circuits apart will take care of most of the problems. Unless you do some very odd things a single solid return plane in your board is the best way to go. On every board you see, that plane will be labeled "ground plane".

Bob

Sent from my iPhone

> On Jun 24, 2015, at 4:54 PM, Neil Schroeder <gigneil at gmail.com> wrote:
> 
> Lets take a hypothetical device, like specifically a PRS10, but could be
> any time/freq device like a PLL or an amplifier or all of these.
> 
> Isolating power supplies with some dexterity can greatly improve noise
> control, and less peformance related, can help us protect very very
> expensive circuits from their less bourgeois support ICs.  Many of us may
> choose to implement them as modular devices for easy relocation or
> replacement.
> 
> Main question: if I want to isolate the quiet and noisy power supplies, or
> just supply them differently, can they share a common return?
> 
> The basic answer has to be yes.  Eventually everything has to get back to
> ground, and the Earth itself is a fair equalizer of all things on it.  I
> can clearly join analog and digital back at the input supply.  But if I had
> them isolated, do those iso grounds have to stay with their iso supplies?
> 
> Take the PRS10 - it has two cables for Vin - power and signal supplies -
> but only one return cable.  Can i isolate, via transformers couplers what
> have you the two supplies yet return the common ground to one or the other?
> 
> The signals handle themselves - they each have a ground wire of their own,
> or does that handle it ?  Are each of those opportunities for a loop?
> 
> I know it'd work with just keeping separate DVcc and AVcc supplies that
> join at some point, and then that return can join at the star ground like
> everyone else.  If you are designing the module, you can handle the
> isolation inside and just have one input supply and return then isolate new
> supplies internally and eliminate the second cable.  Or two in vcc, two out
> gnd.  Those all make sense.
> 
> I think the single return option on the PRS10 is what's hanging me up
> here.
> 
> Does isolation offer any real further benefit than AVxx and DVxx that is
> even worth pursuing outside the extreme use cases?
> 
> Lots of words.  Thanks.
> 
> NS
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