As
you develop an error budget for a circuit or data-acquisition system, you also
must think about the accuracy and resolution of the measuring device.
Unfortunately, some people confuse accuracy and resolution or think they mean
the same thing.
I
use a Fowler digital caliper in my workshop and its digital display indicates
measurements with a resolution of five ten-thousandths of an inch or one
hundredth of a millimeter. But this display resolution can lull you into a
false sense of accuracy. The caliper data sheet notes an accuracy of ±0.001
inch or ±0.02 mm. So, the number of displayed digits doesn't reflect the
accuracy of the instrument. In other words, a display of 0.500 inches most
likely indicates a dimension between 0.501 and 0.499 inches. Think of accuracy
as the "correctness" of measurements.
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In
short, resolution describes how small a measurement an instrument can make. And
accuracy defines how well the instrument makes those measurements. Suppose I
use a 3½-digit DMM with an accuracy of ±1 mV (±1 count) to measure a known
0.1667-V signal. In this case, the meter probably displays 0.167, so the
voltage could range from 0.166 to 0.168 mV. The measurement is not accurate
because the DMM doesn't offer enough resolution. If you want a better
measurement use a 4½-digit DMM with an accuracy of ±100 μV (±1 count) to provide
a reading of 0.1666 to 0.1668V. If that
measurement doesn't come close enough,
you could use a 5½-digit DMM with an
accuracy of ±10 µV (±1 count). But, if you
have a 5½-digit DMM that no one has calibrated in years, you will still see a
5½-digit resolution, but probably not an accuracy of ±10 µV. Always check
manufacturers' data for instrument-accuracy information.
When
you build a measurement circuit from scratch or assemble one from modules you
must budget for errors that can occur within an analog-to-digital converter and
its front-end components such as amplifiers and multiplexers. You can have more
resolution than accuracy, but not more accuracy than resolution.
Although
accuracy and resolution dominate conversations about electrical measurements,
engineers should also know about precision, which determines whether they can
make reliable and repeatable measurements. As shown in the diagrams you can
have precision and accuracy independently, but you should aim to have them
simultaneously.
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