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AD420: Offset Adjustment and Gain Adjustment

Offset Adjustment and Gain Adjustment


The gain adjustment range is fairly easy to calculate given other
information on the data sheet, but the offset adjsut is more difficult
without the internal schematics.

Gain adjust: The 5k RSPAN2 resistor in parallel with the internal 40ohm
resistor will lead to an increase in gain of 0.8%, with the RSPAN
rheostat set to zero. As the rheostat is changed to 500ohms, it will
attenuate the voltage on REFIN against the 30k REFIN resistance, down
to 0.9836, for an overall (RSPAN and RSPAN2 together) factor of -0.85%.
Thus the gain adjust range is about +/-0.8%.

Offset adjust: The OFFSET pin connects through a 100k resistor to an
internal node that is at 1V. Currents forced into or out of this node
are inverted and scaled by a factor of 150 to the output. As the voltage
on OFFSET can be driven from 0V to 5V by the RZERO potentiometer, the
current through the 100k resistor will vary from nominal by +10uA to
-40uA. After the -150 scale factor, this should leat to an offset
adjustment range of -1.5mA to +6mA.