AN2719

ADC errors

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2.2.2

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 10-bit system

means the maximum nonlinearity error may be off by 2/1024 or 0.2%. Note that neither INL

nor DNL errors can be calibrated or corrected easily.

2.3

Offset error

Offset and gain errors can easily be calibrated by the application firmware. First, apply zero

volts to the ADC input and perform a conversion, then the conversion result represents the

zero offset error. Then perform a gain adjustment. A subsequent offset error calibration may

be required. A useful method for offset and gain calibration is the least square method

(which calculates the smallest error in all the used range).

2.4

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 can be observed,

if a full-scale input voltage is applied and the code obtained is 1010 instead of the ideal 1023

(for a 10-bit converter), or if the full-scale code 1023 appears with an input voltage less than

full-scale.

Figure 9.

Offset and gain error

Ideal ADC

Real ADC

ADC output

[code]

110

101

100

011

010

001

000

Input voltage

[V]

max. code

Gain error

offset error