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What is the AD779x gain accuracy?

Question asked by mlitwack on Aug 23, 2011
Latest reply on Aug 26, 2011 by MaryMc

I can't seem to find any information on the gain error for the AD7794 for different gain settings in the datasheet.  I'd like to know what the expected error would be, for example, when measuring a value at gain=4 and then again at gain=8 (assuming of course the value is in range and a full-scale self-calibration is performed before the measurements).

 

The AD7794/AD7795 datahseet in the Calibration section says:

 

To perform an internal full-scale calibration, a full-scale input voltage is automatically connected to the selected analog input for this calibration.

 

The AD779x FAQ in the Self-Calibration section says:

 

The full-scale coefficient is determined by applying a voltage of AVDD/gain to the modulator inputs.

 

So, this would seem to imply that the full-scale calibration is based on the accuracy of the voltage of the AVDD/gain node.  Is this correct, and if so, how accurately is this voltage derived?  (It also seems odd that AVDD/gain is being used instead of Vref/gain, but that's besides the point.)

 

If not correct, how is full-scale self-calibration actually implemented and what are it's dependencies for accuracy?  Are there differences when the in-amp is active?  The FAQ does well in explaining zero-scale, but is very nebulous about full-scale (so, I'm not looking for a cut-and-paste from the FAQ).  A block diagram with switches thrown showing the various calibration states would be extremely helpful.

 

In the same vein, how accurate is the AVDD/6 voltage node?

 

Thanks for any clues,

 

-mark

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