Nonlinear output of ADC AD7643

Hi all,

 

I am using ADC AD7643 with an external reference of 2.5V and testing its 18-bit parallel mode. I calculate the voltage from the ADC digital output bits and compare it with the actual differential input between pin 43 and 39. There is significance difference between the two voltage. Yet the difference diminish at higher input voltage. What could be the problem?

 

 

 

 

Differential Input (mV) Voltage calculated from ADC output (mV)
493 42.5
526 91
571 208
624 315
672 402
738 519
900 782
997 918
1114 1073
1215 1197
1325 1314
1406 1392
1509 1507
1593 1601
1708 1702
1807 1867
1898 1912
2007 1995



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