Practical SCADA for Industry David Bailey - FER-a

SCADA systems, hardware and firmware 23
• Bipolar offset
Similarly, the transition from FS/2-½ LSB to FS/2 (7 FFh to 800 h on a 12-bit
A/D) should occur at ½ LSB below analog common. The bipolar offset
(again, usually adjustable with a trimpot) and the temperature coefficient
specify the initial deviation and the maximum change in the error over
temperature.
• Linearity errors
With most A/D converters gain, offset and zero errors are not critical as they
may be calibrated out. Linearity errors, differential non-linearity (DNL) and
integral non-linearity INL) are more important because they cannot be
removed.
• Differential non-linearity
Is the difference between the actual code width from the ideal width of 1 LSB.
If DNL errors are large, the output code widths may represent excessively
large and small input voltage ranges. If the magnitude of a DNL is greater
than 1 LSB, then at least one code width will vanish, yielding a missing code.
• Integral non-linearity
Is the deviation of the actual transfer function from the ideal straight line. This
line may be drawn through the center of the ideal code widths (center-of-code
or CC) or through the points where the codes begin to change (low side
transition or LST). Most A/Ds are specified by LST INL. Thus the line is
drawn from the point ½ LSB on the vertical axis at zero input to the point 1½
LSB beyond the last transition at full-scale input.
• Resolution
This is the smallest change that can be distinguished by an A/D converter. For
example, for a 12-bit A/D converter this would be 1 /4096 = 0.0244%.
• Missing code
This occurs when the next output code misses one or more digits from the
previous code.
• Monotonicity
This requires a continuously increasing output for a continuously increasing
input over the full range of the converter.
• Quantizing uncertainty
Because the A/D can only resolve an input voltage to a finite resolution of 1
LSB, the actual real-world voltage may be up to ½ LSB below the voltage
corresponding to the output code or up to ½ LSB above it. An A/D’s
quantizing uncertainty is therefore always ±½ LSB.
• Relative accuracy
This refers to the input to output error as a fraction of full scale with gain and
offset error adjusted to zero.