AD7713: deduct the actual value of the constant current source

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Question 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 ? Answer 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.

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AD7713: FAQ