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

Neil Schroeder gigneil at gmail.com
Wed Jun 24 17:54:32 EDT 2015


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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