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AD7730 System Calibration

Question asked by vismay on Oct 15, 2015
Latest reply on Oct 24, 2015 by vismay

Hi All,

 

I am using AD7730 Analog Front End IC in Weighing Application,

where AD7730 samples signal coming from Loadcell.

 

     In my application The material being weighed is filled in an otherwise

empty weigh pan ( having a dead weight or tare of approx.. 40-60 Kg.)

and loadcell monitors weight of this weigh pan (either empty or filled)

 

After performing Internal Zero and Internal Full Sclae Calibration, I am using a 3 step system calibration procedure as below:

 

Step 1: Empty the weigh pan and collect the signal. This step will tare the weigh Pan. This is AD7730 system-zero-calibration.

Step 2: Now a standard weight of bag Nominal Value ( 50Kg ) is placed on weigh Pan and AD7730 is commanded to do system-Full-Scale-Calibration.

Step 3: Again the standard weight is removed from the weigh pan and again the weight of empty weigh pan is inputted.

 

after first 2 steps of system zero and system full scale calibrations,

I manipulate DAC so as to offset weigh pan tare value and get the best zero to full scale range.

Since I need about 5% counts below tare value and approx. 5% counts above

full scale calibration value, I manipulate DAC Register value,

offset Register Value and Gain Register value

during step 2 and step 3 above.

 

Please note, I have carried out various trials and my application is working and i am quite near the results.

However, I do face certain issues, which make me doubt if my calibration method is correct.

 

Are there any application note, where in the Gain and Offset calibration coefficients are manipulated

after system-zero and system-full-scale calibrations such that these manipulated coefficients allow weight range

which is approx. 5% below zero calibration and approx. 5% above the full scale calibration range?

 

Is there any provision in AD7730 such that once we do system low and full scale calibration then it will have

a small guard band nearly 5% of Full scale above Full scale and 5% below Zero scale tolerance so that AD7730 will not

end up in Out of Range Signal ?

 

Or are there any other Analog Devices' IC which has this functionality ?

 

-Vismay

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