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'[OT] Dual supply for mixed PIC/ANALOG project.'
2000\03\31@084825 by Tobie Horswill

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Hi,

   I've just started working on an audio project involving an 16F877 and an
Analog Devices SSM2160 (requiring a  -6V,+6V supply). Any hints on how the
two supply voltages should be generated and routed on the PCB in order to
minimise digital leakage in the audio ? I know this isn't a great idea but
if possible I would be using using a small +5V, +12V laptop computer style
switching supply. How can I best isolate the circuit from the switching
noise ? I'm a programmer kind of guy, not an power-supply engineer so please
forgive my electronics shortcomings... _-.\

Tobie

2000\03\31@093720 by Chris Eddy

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Well, if you must use a switcher, then:

Make a completely solid and seperate ground plane for the analog.  Connect to
the PSU ground at one point only.  Bring +V in through a PI filter, which is a
cap, inductor, and cap.  Place a linear regulator there to drop +v to 6V.  It
will take out frequencies that the PI filter did not.  Hopefully, assuming the
low pass regions overlap.

So you also need a negative voltage.  You can use one of those 7660 style chips,
but many folks find that they have terrible noise.  The other possible solution
is to use an isolated DC/DC converter, either off the shelf or built around a
transformer from PICO or Premier Magnetics.  You can generate both + and - with
one part.  You still have switcher noise, which you try to take out as mentioned
above, but at least you have an isolated ground.

Audio design is a lot harder than many people think.  Good luck.

Chris Eddy

Tobie Horswill wrote:

{Quote hidden}

2000\03\31@113120 by Dan Michaels

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Tobie Horswill wrote:
....
>     I've just started working on an audio project involving an 16F877 and an
> Analog Devices SSM2160 (requiring a  -6V,+6V supply). Any hints on how the
> two supply voltages should be generated and routed on the PCB in order to
> minimise digital leakage in the audio ? I know this isn't a great idea but
> if possible I would be using using a small +5V, +12V laptop computer style
> switching supply. How can I best isolate the circuit from the switching
.....

Chris Eddy wrote:
....
>
>So you also need a negative voltage.  You can use one of those 7660 style
chips,
>but many folks find that they have terrible noise.  The other possible solution
....

Tobie,

As a voice of experience with 766x parts, I can definitely say, "yes,
they generate terrible noise". If using these parts in a pcb layout,
be *certain* to include a ground plane under the part. On a mockup, you
can use aluminum ducting tape underneath [grounded, of course]. Also,
you would want to locate your low-level pre-amps on the other side of
the board, as far as possible from the SMPS and 766x, to minimize pickup
of radiated EMI.

Also, if this is a "hi-fi" audio type project, mixing analog with
noisy digital PICs and SMPSs and 7660s is going to be a real problem,
no matter what you do. Possibly, use separate analog/digital boards
with shielding in between.

best regards,
- Dan Michaels
Oricom Technologies
http://www.sni.net/~oricom
==========================

2000\03\31@120828 by jamesnewton

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see
http://techref.massmind.org/power
http://techref.massmind.org/power/isolation

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