So long as the drift with temperature is the same percentage on both
resistors, in the same direction, and you mount them physically very close
to each other, they will both experience the same changes in temperature and
your circuit will be hardly affected.  At 10 bit resolution on a 5v scale
you can only resolve to 0.1% of scale or 1000ppm.  If you buy some nice 0.1%
resistors with 15ppm / degrees C temperature coefficient, even with one
resistor staying at the same temperature and the other one changing, you
would need to change the temperature by 67 degrees before you see a full 1
bit of error.  I think that's right anyway :-)

Martin

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Lindsay Pallickal
Sent: 08 November 2002 03:05
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE] 0-11v analog input -> 0-5v for pic


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.
Thanks.

Lindsay

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