AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 8

• Manufacturer Part Number: AN2438
• Description: ADC Definitions and Specifications
• Manufacturer: Freescale Semiconductor / Motorola

AN2438/D
Integral Non-Linearity (INL)
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Related to DNL are two critical figures of merit used in defining ADC operation.
These are:
Some literature suggests that a DNL of greater than 1.0 LSB may indicate non-
monotonicity. Non-monotonicity is usually accompanied by large, positive DNL
(>1.0 LSB), although a non-monotonic situation can be coincident with a DNL
of less than 1.0 LSB.
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:
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Conversion
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Freescale Semiconductor, Inc.
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REFL
Function (dotted)
Integral Non-Linearity
Ideal Transfer
For More Information On This Product,
Function (Dashed)
Adjusted Transfer
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Input Voltage in LSB
Missing Codes — An ADC has missing codes if an infinitesimally small
change in voltage causes a change in result of two codes, with the
intermediate code never being set. A DNL of –1.0 LSB indicates the
ADC has missing codes (DNL measured by this definition cannot be less
than –1.0 LSB).
Monotonicity — An ADC is monotonic if it continually increases
conversion result with an increasing voltage (and vice versa). A non-
monotonic ADC may give a lower conversion result for a higher input
voltage, which may also mean that the same conversion may result from
two separate voltage ranges. Often, the transfer function will completely
miss the lower code until after the higher code is converted (on an
increasing input voltage).
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Code INL = V(Current Transition) – V(Baseline Transition)
ADC Definitions and Specifications
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INL = +0.25
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INL = +0.50
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INL = 0.0
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REFH
INL = Max(Code INL)
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
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