I know you definitely need to use external reference. I
remember when I asked question to microchip engineer
that what  voltage range can input have when using
external reference they said as long as do not go
over reference voltage we are ok. you might need to
verify this again. In this case your reference voltage should be
18 volts  RA3  and  "FF" you will have on adres register.

Andre Abelian



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