I agree with Scott's elaboration on my caution about the ambient temperature
effects on the resistors of the circuit.  I used the resistors as an example
but the op-amp may have as much error contribution through variation of input
offset voltages with temperature.  For high accuracy, all components need to
be selected for temperature stability.

Also, Scott's suggestion to sample the power supply voltage in addition to
the signal is a good one if you have an extra ADC.  Another alternative is to
use a band gap reference with good temperature stability to generate the
subtraction voltage.


Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  Mcorio@aol.com

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