Analog-to-Digital Converter (ADC) Integral non-linearity (INL) is defined as the maximum vertical difference between the actual and the ideal curve. It indicates the amount of deviation of the actual curve from the ideal transfer curve.
INL can be interpreted as a sum of DNLs. For example, several consecutive negative DNLs raise the actual curve above the ideal curve as shown in the figure below. tthe INL in this case is positive. Negative INLs indicate that the actual curve is below the ideal curve. This means that the distribution of the DNLs determines the integral linearity of the ADC. The INL can be measured by connecting the midpoints (end-point method) of all output steps of actual ADC and finding the maximum deviation from the ideal curve in terms of LSBs. From the figure, we can note that the maximum INL is +0.75 LSB. The maximum positive and negative INL are usually specified for stated operating conditions.
Another method used to determine INL is the best-fit method. The best-fit response is found by manipulating the gain and offset for the measured transfer function, comparing against a linear transfer function, then balancing the total positive and negative deviations.