At 10:05 PM 07/11/2002 -0500, you wrote:
>I am a student using a PIC 16F877 in a project and would like to read an
>analog input from an accelerometer setup that varies its output from 0-11v.
>This voltage is put out by a signal conditioner made by the manufacturer of
>the accelerometer so I have little control over it. Now I know there are
>vref pins I can use, but some analog inputs will be in the 0-5v range and
>I'd rather not lose precision on them by changing the voltage reference to
>accomodate the range of the accelerometer. How can I step down the 0-11v to
>0-5v? Can I use two high high value resistors to divide the voltage down? I
>am considering that approach but am afraid of the effects temperature will
>have on the resistors and thus the readings, since I am told that resistors
>are prone to drift with temperature. The accelerometer will be used
>outdoors, inside a model airplane, where temperature could vary
>considerably. Also, having little background on EE, I am not sure what other
>effects might follow from using the resistors. Any thoughts or other
>approaches? I know little about EE, so bear with me if this is too basic.

This is actually a common problem.  In your specific case, this is not too
tough.

To ensure that you get a good output from this sensor, an op-amp is a nice
way to buffer a voltage.

I recommend using a voltage divider, and a rail-to-rail op-amp, configured
as a voltage follower.

This should look something like this (copy into Notepad or similar):


Vin (0-11V)
------------+
             |
             \
             /
             \R1
             /
             \            +5V
             /             |
             \             |
             | (0-5V) Vb |\|         |
             +-----------|+\    Vout |
             |           |  >--+-----|ADC input
             \       +---|-/   |     |
             /       |   |/|   |     |
             \       |    ---  |
             /       |    ///  |
             \       +---------+
             /R2
             \
             |
            ---
            ///

Use Vout = Vb = R2/(R1+R2)
to determine R1 and R2.

Hope this helps,

--Brendan

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