Integral nonlinearity (acronym INL) is a commonly used measure of performance in digital-to-analog (DAC) and analog-to-digital (ADC) convertors. In DACs, it is a measure of the deviation between the ideal output value and the actual measured output value for a certain input code. In ADCs, it is the deviation between the ideal input threshold value and the measured threshold level of a certain output code. This measurement is performed after offset and gain errors have been compensated.
The ideal transfer function of a DAC or ADC is a straight line. The INL measurement depends on what line is chosen as ideal. One common option is the line that connects the endpoints of the transferfunction, in other words, the line connecting the smallest and largest measured input/output value. An alternative is to use a best fit line, where one minimises the average (or alternatively the mean squared) INL.
While the INL can be measured for every possible input/output code, often only the maximal error is provided when reporting the INL of a convertor.
Formula
For the line through the endpoints, the INL of a DAC is
where
is the slope of the line through the end points, and
is the output voltage at code c. This assumes that the minimum code is 0. This INL is measured in volts; one can divide it by the ideal LSB voltage to get the measurement in LSBs..