At 02:20 PM 3/2/01 -0800, you wrote:
>I am attempting to do a A/D conversion to a signal ranging from -1 VDC to
+5.5. VDC.  I am using a PIC16F877.  I understand to need to use the VREF,
+VREF, and -VREF pins in order to do this.  The problem is that I have not
found a good description of how to use/apply these pins.

Have you had a look at the block diagram in section 11 of the data sheet?
And table 15-12 ?

You won't be able to directly cover that range with the A/D, you may need
an op amp
or just a couple of resistors to get it within the range of 0 < Vin < Vcc.


(Once it's within that range, you can decide where you want the A/D
converter to operate,
within reason. Vref- should be between about Vss and Vref+ -2V. There has
to be at
least 2V between Vref+ and Vref- and Vref+ should not be higher than Vdd.)

For example:

           +5 = Vdd = Vref+
                |
                R1
                |
Input -- 10K0 --x->>  Analog in 0..5V
-1 ~ 5.5V       |
                R2
                |
               0V = Vref -

By inspection (at the limits) you can easily see that R1 must
be 50 * 10K and R2 must be 10 * 10K0

So, closest E-96 values, 10K0, 49K9, 100K

(note that you should have a fairly low source impedance if you use this
method, depending on the accuracy you need, less than 50 Ohms if you need
the full 10 bits - if it's higher but *known* and *constant*, you can just
subtract it from the 10K0).

Best regards,


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