I doubt you will get a logic chip that can drive a -+5 voltage swing.  Why
not stick with the TI chip design, and invert 8 of the outputs into -5V
using a couple of chips such as the LMC7660 switched capacitor voltage
converter - this chip is designed to switch a +v to a -v with just two
capacitors?  Just a suggestion ...  Trouble is - this design comes up to at
least 4 IC's and I am wondering if that would be any cheaper than using 16
FETS in a cascaded Mosfet switch design.

Regards
Ian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Johnathan Corgan
Sent: Wednesday, 5 February 2003 12:10 pm
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Converting PIC port output to +-5V @ 20 ma drive


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:

http://www-s.ti.com/sc/psheets/scds017l/scds017l.pdf

I've also thought of using a comparator, with +-5V supplies and an input
reference set between the PIC high and low output voltage.  The LM339
can only sink 16 ma maximum, typical is 6 ma, and in general, 20 ma
seems to be on the high end of the current sinking capability of common
comparators.

Any suggestions?  Am I completely missing some other, simpler approach?
  Does someone make an "Octal +-5V input swing analog multiplexer using
CMOS level inputs" part that I can order from Digikey?

Thanks, Johnathan

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