Integral Non-Linearity (INL) is defined as the sum from the first to the current conversion
(integral) of the non-linearity at each code (Code DNL). For example, if the sum of the DNL up to a particular point is 1LSB, it means the total of the code widths to that point is
1LSB greater than the sum of the ideal code widths. Therefore, the current point will convert one code lower than the ideal conversion.
In more fundamental terms, INL represents the curvature in the Actual Transfer Function relative to a baseline transfer function, or the difference between the current and the ideal transition voltages. There are three primary definitions of INL in common use. They all have the same fundamental definition except they are measured against different transfer functions. This fundamental definition is:
Code INL = V(Current Transition) – V(Baseline Transition)
INL = Max(Code INL)
The three primary definitions are:
- INL, Adjusted INL, or Endpoint INL — The current transition voltage is compared to the corresponding transition voltage on the Adjusted Transfer Function. This is a useful indicator of the best the ADC can do if the endpoint non-linearities (Zero- and Full-Scale Errors) are measured and trimmed out.
- Unadjusted INL — The current transition voltage is compared to the corresponding transition voltage on the Ideal Transfer Function. This is a measure of the total error except for Quantization Error.
- Best-Fit INL — The current transition voltage is compared to the corresponding transition voltage on the Best-Fit Transfer Function. This will usually give a balanced positive and negative error across the entire curve so the results look very optimistic, but since it is difficult to obtain the Best-Fit Transfer Function in application, it is not a very useful measure. Unfortunately, this is what many evaluation packages (hardware and software) measure.
There are some related definitions to Total Unadjusted Error that vary slightly in definition. These are:
- Total Error is the same as Total Unadjusted Error, but the term is misused in several ADC references and is therefore misleading. The less ambiguous term Total Unadjusted Error is preferred.
- Total Adjusted Error is the difference between the Actual and Adjusted Straight-Line Transfer Function, accounting for INL plus Quantization Error. This term is redundant and potentially confusing with respect to Total Unadjusted Error and will not be used.
- Code Error is the error between the ideal code and the current code. This is the only figure of merit that measures by the quantized output.
Instead of voltage. The code error is the Total Unadjusted in LSB, rounded to the nearest integer.
References:
- J. Feddeler and Bill Lucas, 8/16 Bit Division Systems Engineering, Austin, Texas, Aplication Note AN2438/D 2/2003, Frescale Semiconductor, Inc, Motorola 2003, www.freescale.com
- http://en.wikipedia.org
