[sdiy] Replacing DACs

[Harry Bissell Jr]

CHHA... and I'm not just the president...
I'm also a MEMBER  :^P

Every "input" is driven by another output.

they can be shorted only in a few cases...

1) some current limit exists in the driver chip
(often, shorting multiple lines from the same chip
will overheat and kill the chip even if individual
circuits are 'current limited')

2) The driver outputs can be forced to a high
impedance (three-state outpus used in computer
busses etc)

3) Open Drain or Open Collector chips - these circuits
use a passive pull-up and the outputs switch to
ground.
shorting these creates a "wire-or" condition... either
the output, OR the wire you added... cause the low
condition.

You might add gates to control the unwanted DAC
bits...
the simplest method would be to use diodes to form
passive or-gates... or add the series resistors as I
suggested.  In this case if you short at the input to
the DAC... the driver is protected by the series
resistor.  This would work if the DAC had a high
enough
input impedance that you could add the series resistor
without problems.  

I'd say that for a 5V system... you could probably add
a 500 ohm to 1K ohm resistor and not have any trouble.

H^) harry



--- john mahoney <jmahoney at gate.net> wrote:
> On behalf of the Clueless Hardware Hackers
> Association (founded just a
> second ago), thanks! :-)
> 
> But, wait... I was thinking through an inverter
> again. This is digital to
> analog... So, how about grounding the input pins of
> the undesired low bits,
> thus forcing their ouputs low. This assumes that the
> DAC inputs accept
> signals of either High or Ground ("higher ground,"
> ha ha).
> --
> john
> 
> 
> ----- Original Message ----- 
> From: "Harry Bissell Jr" <harrybissell at prodigy.net>
> To: "john mahoney" <jmahoney at gate.net>; "nN AAt e e"
> <timexheater at comcast.net>;
> <synth-diy at dropmix.xs4all.nl>
> Sent: Friday, March 12, 2004 2:23 PM
> Subject: Re: [sdiy] Replacing DACs
> 
> 
> > shorting bits to ground would be bas,
> electrically.
> > you probably will damage the gear.
> >
> > It might be possible to add series resistors to
> the
> > low bits... preventing the driver chip from frying
> > from
> > the direct short... many other ways exist
> >
> > H^) harry
> >
> >
> > --- john mahoney <jmahoney at gate.net> wrote:
> > > nate,
> > > You needn't replace a DAC just to drop the bit
> > > resolution. Grounding the
> > > desired low bits should suffice. To make a
> 12-bit
> > > DAC act like an 8-bitter,
> > > just ground the 4 low bits (or switch them out
> of
> > > the circuit).
> > >
> > > Logically, the above is accurate. Electrically?
> I'm
> > > not responsible if you
> > > damage your gear.
> > > --
> > > john
> > >
> > >
> > > ----- Original Message ----- 
> > > From: "nN AAt e e" <timexheater at comcast.net>
> > > To: <synth-diy at dropmix.xs4all.nl>
> > > Sent: Friday, March 12, 2004 4:49 PM
> > > Subject: [sdiy] Replacing DACs
> > >
> > >
> > > > i was wondering if anyone has taken some older
> > > [IE. 80s Digitech and other
> > > > misc.] digital effects and replaced the DACS
> with
> > > similar ones but of say,
> > > > lower bit rate. i know that you would probably
> > > need to hack or replace
> > > clock
> > > > signals, and then wire everything back up to
> > > compensate for the different
> > > > pins. variable rate and depth would be fun.
> > > >
> > > > it just seems like a fun idea, and a little
> more
> > > versitile than buying a
> > > > good bit grunger/mangler/sonic distortion.
> > > >
> > > > - nate
> > > >
> > > >
> > >
> >
>