You can make magic smoke if you use a Vref that large.  If you need an
absolute accurate A/D then buy a reference chip that creates either
4.096V or 5.0V.  To measure your voltages use voltage dividers to bring
the voltage down to a value between 0 and +5 or 4.096V

John



Wireless CAN with the CANRF module.
http://www.autoartisans.com/documents/canrf_prod_announcement.pdf
Automation Artisans Inc.
Ph. 1 250 544 4950


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Michael A. Powers
> Sent: Saturday, June 01, 2002 3:57 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: ADC Operations at voltages > Vdd
>
>
> Hi All,
>
> I'm using the PIC 16F877's ADC unit to determine voltages,
> and I want a
> Vref+ of 24V.  Is this possible?  The datasheet seems to say
> no, but I'm not
> sure.  The maximum value of Vref+ is stated as AVdd + 0.3,
> but I'm not sure
> what "AVdd" is.  What is meaning of the "A" in AVdd?  Then
> again, the spec
> also says that Vref+ - Vref- can be no more than Vdd + 0.3.
> Correct me if I
> am wrong, but it seems that I cannot use a 24V input direct
> to the ADC.  I
> suppose I'll have to use a voltage divider to get the input's max to
> something within specifications, but is there a better way?
>
> Thanks,
> -Mike
>
> --
> http://www.piclist.com hint: PICList Posts must start with ONE topic:
> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
>
>
>
>

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads