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# AD7713: deduct the actual value of the constant current source

### Q

For AD7713: Is it possible to deduct the actual value of the constant current
source  (set value = 200µA) from the zero and full scale calibration
coefficients ?
Example
Given:  zero reference resistance        100 Ohms     system calibration
step one
gain reference resistance 5590.50 Ohms      system
calibration step two
Gain = 1
Ref Voltage                     2.5 Volts
Zero calibration coefficient    0x3BEADB  (dec 3926747)
FS calibration coefficient     0xE4288E        (dec 14952590
)
the measured actual value of the constant current is 220.19 µAmps.
Following the data sheet rev. C p 17 the result is obtained by
subtracting the zero cal.
value from the measured value and then multiplying with the FS cal
coefficient.
Result= (measured value[counts]  - ZERO cal) * FS
000000<>((100Ohms * 220.19µA)/2.5 Volts* 16777215-  3926747) *  14952590
FFFFFF<>((5590,5 Ohms * 220.19µA)/2,5 Volts * 16777215 - 3926747) *
14952590
Do we have erroneous assumptions or is this simply not possible ?

### A

The 200uA current sources included on the AD7713 have quite wide tolerances on
them at +/-20% and the matching between them is about +/-1%. For most
applications it is not necessary to know the absolute value of the current
source. For example, if you have a sensor whose output is a change in
resistance, you can use a low TC precision resistor in series with the sensor,
then use the two differential channels of the ADC to measure the voltage across
the precision resistor, and the voltage across the sensor. Since the same
current is flowing through both the precision resistor and the sensor, the
whole thing is ratiometric and the absolute value of the current source is not
required.

I think that what you want to do is compensate for the tolerance of the current
sources by adjusting the calibration co-efficients. In most cases this is not
necessary, as often you can make the system ratiometric by looking again at the
measurement that you wish to make.

It is possible to adjust the cal co-efficients but this is not as straight
forward as you describe.