Richard Prosser wrote:

> Depending on accuracy & if calibiration is likely to be an issue why not use
> a tri-state port to measure the voltage,
> A resistor connects from the voltage source to the port which is also
> connected via a capacitor to ground. A diode to the supply rail protects the
> port again overvoltage.

As stated before, if you're not too concerned with absolute accuracy, I would go
with the voltage divider network.

Vi ---------+           Vcc
            |            | +                    +-------
            R1           Dprot(just in case)    |  PIC
            |            | -                    |
            +------------+----------------------| A/D Port Pin
            |            | +                    |
            R2           Dprot                  |
            |            | -                    |
            GND          GND                    +-------

You can choose R1 and R2 (R2<R1) so your expected voltage
swing on the output of the amp (7-18V) translates fairly well
to within 0-5V for the input of the PIC.  The PIC A/D is
straightforward (and has pretty good working resolution).  You would still have
to implement a lookup table or calculate the
output voltage from this conversion.

-Vincent Deno




> Then by switching the port from a "0" output to a (high Z) input, the time
> taken to charge the cap to the threashold (input = 1) voltage can be used to
> lookup or calculate the required voltage. The input is then changed to an
> output at 0 again to discharge the cap. (though a limiting resistor)
> Likely problems would be the variation in threashold voltage and capacitor
> tolerances etc. but dependent on accuracy required it could be a "quick and
> dirty" solution.
>
> Richard
>
> (rprosser@swichtec.co.nz)
>
> -----Original Message-----
> From: Vincent Deno [mailto:denovj@EMAIL.UC.EDU]
> Sent: Thursday, May 13, 1999 3:40 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: High Voltage PIC A/D
>
> There are a lot of ways to do this (as with anything else).  But, are you
> allowed to sacrifice amplifier performance?  Adding additional circuitry to
> the output of your amp could significantly hinder its response.  Also, must
> this circuit be powered from a single +5V rail?
>
> > Steven Kosmerchock wrote:
> >
> > > Friends,
> > > I have a project where I need to read an analog voltage of
> > > +7V-+18V. It is an internal power monitor of a RF amplifier.
> > > I am using it in a test fixture and was hoping of some way
> > > to read the voltage to know how many watts are at the
> > > output of the amp, without using a power meter.
> > > Does anybody know of an A/D converter that
> > > goes that high? Does the PICs internal A/D go that high?
> > > Any ideas?? Thanks in advance.............................
> > >
> > > Best regards,
> > > Steven
> > >
> > > Steven Kosmerchock
> > > Engineering Technician/Student
> > > CELWAVE
> > > Phoenix   Arizona   USA
> > > Email:   steve.kosmerchock@celwave.com
> > > www.geocities.com/researchtriangle/lab/6584