My first thought would not be to use a processor at all.  I would run
the 50 inputs to 50 open collector buffers pulling down a resistor
network.  Something like a R2R network but it might be simpler as you
don't have to distinguish when multiple lines are low together.  Finally
use an op amp to buffer and filter the output.

Sherpa Doug

> -----Original Message-----
> From: tim [mailto:stm800@CITY-NET.COM]
> Sent: Wednesday, August 29, 2001 8:37 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: PIC UNUSUAL D/A APPLICATION
>
>
> I have 50 seperate logic lines  that switch in sequence from
> # 1 to # 50 then reverse from # 50 to # 1
> at any one time only one line is in a low state  and all
> others are high state. what I need is an analog output
> voltage generated from the pic in response to which switched
> line is enabled.
> example:
>
>  max reference voltage output is 1 volt at line 50 switched on.
>  half of the reference voltage is 500 mv at line 25 switched on.
>  each switched line represents a 20 mv  change in output voltage.
>
>  these parameters are not set in stone and the analog
> reference voltage could
> change if it made code and chip selection easier.
>
> any thoughts on which pic chip to use  as i have not bought
> any chips yet.
> is there a simple  mixed signal  or  d/a chip that will do
> this instead of using a pic chip?
> it seems that this type of digital line switching (single
> line enabled only )
> to generate an analog voltage is really not what the D/A
> chips do that are
> presently out there on the market. well as easy as this
> application seems to be
>  i want to ask before  i begin  re-inventing  sliced bread....
>
> thanks alot  for reading this and all comments and
> suggestions welcome         tims 800
>
>
>
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