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.
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