How about a heavily on diode through a resistor divider?


    +5V
     |
     |
     >
    <
     >
    <
     >
    <
     |
     +-----+ 0.65V
     |     |
     |      >
     |     <
    ---     >
    \ /    <  <----- Vref
    ---     >    |
     |     <    --- small C to reduce dynamic
     |     |    --- source resistance.
     |     |     |
     +-----+-----+
     |
   -----
  / / /

Shown here Potentiometer, instead of two resistors because I got lazy.

It would'nt be all  that accurate, due to temperature drift, but it might be
good enough for what you want...  And besides, the on voltage of a signal
diode doesn't drift all *that* much... well, best of luck.

--Brendan

----- Original Message -----
From: "Alvaro Deibe Diaz" <adeibe@CDF.UDC.ES>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, July 02, 2002 10:11 AM
Subject: [PIC]: Precision 0.5V reference for AD


> Hello.
>
> I am planning to read some 0.5V to 4.5V sensor
> voltages, and need good 0.5V and 4.5V precission
> references. I plan to use Analog Devices REF194
> to get the 4.5V, but don't know how to get the
> 0.5V without using resistors, op-amps, etc.
>
> Any ideas?
>
> Thanks in advance,
>
> Alvaro Deibe.
>
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