If what youre looking for is a 0-5v to 4-20mA converter why dont you try a
dedicated
chip?
xtr110 from burr brown fits the bill.
http://www.bbrown.com/Products/DataSheets/XTR110.html

Justin Grimm




Thomas McGahee wrote:

> Andrew,
>
> One method you could use to control the dimmer that requires a current
> sink is to let the PIC control a voltage to current converter. Let's
> assume you want an output of +4 volts to generate a current sink of
> 20 ma. The basic current sink looks like this:
>
>
> ----------
> > From: Andrew Yalowitz <AndrewY@STAGESOUND.COM>
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: Interfacing PIC to industrial automation
> > Date: Wednesday, October 28, 1998 8:31 AM
> >
> > The fact that the dimmer generates the current to be sunk totally threw me
> > for a loop. Until now, **every** dimmer system that I have seen or read
> > about (admittedly theatrical and architectural)  has looked for a voltage,
> > usually 0 - 10v
> > When I went out to take a look at the dimmer in question, I measured the
> > voltage and it looked like it was 0 to 24v. When my interface box (which
> > generates 0 - 24v) didn't work, I did more research and discovered that the
> > dimmer was actually generating the current which was being sunk by the
> > controller.   A friend told me about a Programmable Logic Controller that
> > could sink the 4 to 20 mA that I need, and I was wondering how it did this
> > so I could replicate it with my pic.
> >
> > Thanks for your advice!
> >