At 05:10 PM 2/4/2003 -0800, you wrote:
>I hope this isn't embarrasingly simple.
>
>I need to have a PIC port drive a signal, referenced to ground, at 5V or
>-5V, either sourcing or sinking up to 20 ma (per bit, 8 bits).
>Furthermore, it would be prudent, but not necessarily required, to have
>galvanic isolation between the input and output, as the output may be
>subject to static electricity.
>
>This would be easy to build with discrete parts, using two FETs arranged
>similarly to a CMOS output, but with the lower one connected to -5V
>instead of ground.  Depending on which FET is conducting, the driver
>output gets "hooked" to +5V or -5V.  (And opto FETs could be used for
>isolation if needed.)
>
>To save parts count (16 FETs!) and $$$, I'd rather go for an integrated
>approach.  There are many quad 1:2 analog multiplexers out there, which
>is what the discrete circuit above really is.  A couple of these could
>switch eight outputs between +5V and -5V inputs very easily.
>Unfortunately, none seem to be able to handle the -5V swing on an analog
>input port.  The TI SN74CBT3257, for example, has absolute max ratings
>of -0.5V to 7V analog input swing:


You could consider 3 74HC4053s. No isolation, but < 80 cents total.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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