> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of alan smith
> Sent: 26 February 2010 15:47
> To: Microcontroller discussion list - Public.
> Subject: [EE] poor mans negative rail generation
> 
> OK, so I have a part that requires a +/-12V on it, and I do have +24V
> available.  So, a poor mans method would be to use a voltage divider
and
> generate a +/- 12V and use the centertap for the ground reference.
The
> 'ground' or virtual ground, would be completely isolated from the rest
of
> the circuit of course.  The kicker is that there are digital signals
(SPI),
> referenced back to the board ground, and the part requires a digital
3.3V
> for this interface (actually can be 5V).  I could generate a local 5V
from
> the +12, referenced to the virtual ground again but the only way I can
see
> to get the SPI is to optically/transformer couple it into this
section.  I
> believe ADI makes a part that can do this, and its not the fact that
its
> another part, its also room on the board for another part.
> 
> Short of this idea, what other methods to generate a split supply from
the
> +24V ?  I have very little room for much of anything, so it has to be
> small and pretty sure not enough room for a negative buck switcher
unless
> very tiny.  Total current isnt going to exceed 1A

You need to make absolutely sure that there will be no significant
digital currents introduced into your virtual ground as any SPI noise on
this will come straight out of the device as a voltage or current.  

You may be able to use current mirrors to do the voltage translation;
they are often very handy for this kind of thing IME.

Mike 

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