AN3137 datasheet(17/42 Pages) STMICROELECTRONICS

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AN3137

ADC errors

Doc ID 16983 Rev 3

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Figure 8.

DNL: missing code

3.2.2

Integral nonlinearity

Integral nonlinearity (INL) is defined as the integral of the DNL errors. So, good INL

guarantees good DNL. The INL error shows how far from the ideal transfer function value

the measured converter result is. For example, an INL error of ±2LSB in a 12-bit system

means the maximum nonlinearity error may be off by 2/4096 or 0.05%. Note that neither INL

nor DNL errors can be calibrated or corrected easily.

3.3

Offset error

Offset and gain errors can easily be calibrated by the application firmware.

First, connect ADC input to VREF-/VSS (low potential of reference voltage) and perform a

conversion. Then the conversion result represents the zero offset error. Then a gain

adjustment is performed. A subsequent offset error calibration may be required.

3.4

Gain error

Gain error is defined as the full-scale error minus the offset error. Full-scale error is

measured at the last ADC transition on the transfer-function curve and compared to the

ideal ADC transfer function.

Gain error is easily corrected in firmware with this linear function:

y = (k1/k2).x

where k1 is the slope of the ideal transfer function and k2 is the slope of the measured

transfer function.

Offset error and gain error can decrease dynamic range. For example this is observed if a

full-scale input voltage is applied and the code obtained is 4092 instead of the ideal 4095

(for a 12-bit converter), or if the full-scale code 4095 appears with an input voltage lower

than the full-scale voltage.

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