What is the value or weight of an LSB in the offset and gain register of the AD779x ?
1 LSB of the offset register is equivalent to approximately 1.3 LSB of the data register, assuming the nominal full-scale coefficients are present. The exact value varies slightly from part to part, and the ratio changes if the full-scale register coefficients are modified.
The exact ratio can be derived by dividing the value in the full-scale register by 0x400000. This gives a value close to 1.33 with the nominal full-scale coefficient of 0x555555. But if the full-scale register is modified by the user, the ratio changes. This occurs since the offset removal is performed before the gain scaling when the ADC is adjusting the converter output.
The full-scale register can be interpreted as a multiplication factor, whose value equals (full-scale coefficient/0x400000.) Since the scaling is done after the offset register is removed, the relative weight of an offset register LSB is different to a data register LSB.
The nominal value of 1.3 for the gain scaling is because the input signal is attenuated by 3/4 as part of the ADC conversion. A 4/3 scaling is then required to digitally compensate for this. (This is normally transparent to the user; it’s only when manipulating calibration values that this can become apparent.)
The signal flow can be viewed as [Input Signal] -> [PGA] -> [Attenuation by 0.75] -> [ADC Conversion] -> [Subtract Offset] -> [Scale by FS/0x400000] -> [Data Register]
If the system offset calibration is done using the ADC’s system zero-scale calibration mode with the systems zero-scale point applied as input voltage, then this scaling factor does not need to be accounted for, since the ADC calibration routine will write the correct value into the offset register. It’s only if calibrations are done using regular conversions (that is, a result is written into the data register) that a scaling factor is required before writing into the offset register.
For the full-scale coefficient or full-scale calibrations, it is a simple scaling coefficient, so to increase the gain of the ADC by 10%, the full-scale coefficient needs to be increased by 10%.
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