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ADuC706x - Full Scale & Zero Scale Ranges with External Reference

Question asked by jbuckf50 on Jul 8, 2016
Latest reply on Jul 26, 2016 by barryzhang

Hello.  I'm using an ADuC7061 with an external reference. 


External reference:  460mV differential on top of approximately 1.25V common mode. 


Signal:  Up to ~125mV on top of ~1.5V common mode.


My gain = 4, AMP_CM = 0.


The noise performance of this is great but I'm having trouble getting rid of a ~0.1% offset.  I thought implementing zero & full scale cal would do the trick but I'm seeing even more issues there. 


For zero scale, I replace my signal with about 1mV; common mode is unchanged.


For full scale, I jumper over (via analog switches) the reference to the analog inputs.  


I'm picking up a ~4% error running zero scale cal, then a big error (25%) running full scale.


Looking at the data sheet, I see that my nominal signal ranges fall within the specs, but the full scale cal violates the max input specs.  However, the datasheet indicates one should replace the input with 'an external full scale voltage' for FS cal. 


Does the external full scale voltage have to equal the reference voltage, or can it be any level I want (that is less than the reference)?


Above, I mentioned my signal is ~125mV; the real range of interest I care about is about 80mV to 125mV.  Can I just use 80mV for zero scale cal and 125mV for full scale?  The datasheet is somewhat vague in this regard.  I consulted the AD7794 datasheet as well but it didn't fully address my question.


Thank you for your time.