AnsweredAssumed Answered

AD7794 incorrect internal full-scale calibration

Question asked by MrWinter on Oct 23, 2011
Latest reply on Dec 19, 2011 by MaryMc

Hi,

 

In my thermocouple application, I carry out internal zero and full-scale calibration on the AD7794 and have a gain error of around 0.5%.

 

When I use default calibration values the error reduces to around 0.1% (part of which is most likely offset error).

 

I am using the settings below for both the calibration and the conversion (apart from changing the mode) My internal calibration values are

zero: 8386977

gain:5598716.

 

My VDD is 3.3V and I have the bypass caps as recommended in the datasheet.

 

Can anyone tell me if the calibration values above look bad (too high???), or if the calibration settings used below are in some way not appropriate for the calibration routine?

 

 

 

 

 

//---------------------- AD7794 register values following conversion -------------------------//

 

Status: 00001001  =>Ready, channel setting 001 (AN2-/AN2+)

 

Mode: 011 0 00 1 0 00 0 0 1111

 

Power down mode (following single conversion) (this is 010 = idle after the calibration).
Power switch open
Nothing
AMP-CM
Nothing
Clock internal 64 kHz
Nothing
Chop enabled
Update rate is 4.17Hz

 

Config: 10 0 0 1 110 10 0 1 0001

Bias connected to AIN2-
Burnout detect disabled
Bipolar coding
Boost on
Gain 64x
Internal 1.17V reference
Ref det disabled
Buffered
Calibration pair 1 (for AN2-/AN2+)

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