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98640A Analog Input Interface
C. The correct sign for PGA_ Offset_X_ 512 is the same as that you
read in step 2A above. Correct the sign of PGA_Offset _X_SI2, if
necessary.
3. Application of offsets
A. Read the input channel you are interested in. Save the Uncalibrated
_Input_Reading and the gain (G).
B.
Subtract ADC _ Offset from IUncalibrated_Input_Readingl (ab-
solute value of Uncalibrated_Input_Reading). The difference is the
IPartly _ Corrected_Readingl.
C. The sign of the Partly _Corrected_Reading must be the same as the
Uncalibrated_Input_Reading. Correct it, if necessary.
Now, to use the PGA_ Offset to change Partly _Corrected_Reading to Calibrated_Reading we need
gain (G).
D. Multiply G by PGA_Offset and then truncate the product to an in-
teger. This corrected product is the Derived_PGA_ Correction.
E. Subtract
Derived_PGA_ Correction
from
Partly
_ Corrected_Reading. The difference is Calibrated_Reading. Since
this was a subtraction of signed numbers (not. absolute values) the sign
(polarity) will be correct.
NOTE
Calibrated_Reading may be less than, equal to, or
greater than Uncalibrated_Input_Reading.
With the corrections done,
we can convert Calibrated_Reading to a voltage.
Multiply
Calibrated_Reading by the lsb value for the gain used for the reading. The product is in volts with
the sign indicating the polarity.
Quick Calibration
Read the paragraphs and note under the heading "Conditions" in the explanation of a detailed calibra-
tion. That information applies to this discussion also.
To make a quick calibration you will need to:
1. Measure the gross offset.
2. Apply the offset to a reading from an input channel
4-5
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