What is the relationship between analog input and the calibration coefficients of the AD779x ?

What is the relationship between analog input and the calibration coefficients of the AD779x ?

Hi Robk.

Just a clarification.

1) Those formulas are related to internal calculation steps done by AD779x?

If the response for question 1) is no, this means that I need to use those formulas instead of ones depicted on Data Output Coding" section of AD779x datasheet? Is it correct? So, I have another question related to that:

2) They are valid both system and internal calibration?

Thanks in advance for your support.

Regards

Tino

Hi Tino,

the equations listed in the datasheet assume no offset and no gain errors. The datasheet equations give the ideal analog input to digital output relationship. So, after completing a calibration which will minimise the offset and gain errors, the equations in the datasheet can be used.

The equations listed in this thread show how the value in the offset and gain registers affect the analog input to digital output process. The equations are valid for both internal and system calibrations.

Regards,

Mary.

The equations for the output digital words with the applied analog input voltage follow:

In unipolar mode, the relationship is

Data = [(0.75 × VIN × Gain/VREF) × (223) – (Offset_Reg – 0x800000)] × Full_Scale_Reg/0x400000 × 2

In bipolar mode, the relationship is

Data = [(0.75 × VIN × PGA_Gain/VREF) × (223) – (Offset_Reg – 0x800000)] × Full_Scale_Reg/0x400000 + 0x800000

Where

Data = digital conversion

VIN = applied analog input voltage

Gain = PGA gain setting

VREF = reference voltage

Offset_Reg = value contained in the offset register

Full_Scale_Reg = value contained in the full-scale register

The nominal values of Offset_Reg is 0x800000 and of Full_Scale_Reg is 0x555555.The 0.75 number will vary slightly from part to part because of manufacturing tolerances.