Accuracy, Resolution, Precision

Asyou develop an error budget for a circuit or data-acquisition system, you alsomust think about the accuracy and resolution of the measuring device.Unfortunately, some people confuse accuracy and resolution or think they meanthe same thing.

Iuse a Fowler digital caliper in my workshop and its digital display indicatesmeasurements with a resolution of five ten-thousandths of an inch or onehundredth of a millimeter. But this display resolution can lull you into afalse sense of accuracy. The caliper data sheet notes an accuracy of plus or minus 0.001inch or plus or minus 0.02 mm. So, the number of displayed digits doesn't reflect theaccuracy of the instrument. In other words, a display of 0.500 inches mostlikely indicates a dimension between 0.501 and 0.499 inches. Think of accuracyas the "correctness" of measurements.

Inshort, resolution describes how small a measurement an instrument can make. Andaccuracy defines how well the instrument makes those measurements. Suppose Iuse a 3.50-digit DMM with an accuracy of plus or minus 1 mV ( plus or minus 1 count) to measure a known0.1667-V signal. In this case, the meter probably displays 0.167, so thevoltage could range from 0.166 to 0.168 mV. The measurement is not accuratebecause the DMM doesn't offer enough resolution. If you want a bettermeasurement use a 4.50-digit DMM with an accuracy of plus or minus 100 I.25V ( plus or minus 1 count) to providea reading of 0.1666 to 0.1668V. If that
measurement doesn't come close enough,you could use a 5.50-digit DMM with an

accuracy of plus or minus 10 muV ( plus or minus 1 count). But, if youhave a 5.50-digit DMM that no one has calibrated in years, you will still see a5.50-digit resolution, but probably not an accuracy of plus or minus 10 muV. Always checkmanufacturers' data for instrument-accuracy information.

Whenyou build a measurement circuit from scratch or assemble one from modules youmust budget for errors that can occur within an analog-to-digital converter andits front-end components such as amplifiers and multiplexers. You can have moreresolution than accuracy, but not more accuracy than resolution.

Althoughaccuracy and resolution dominate conversations about electrical measurements,engineers should also know about precision, which determines whether they canmake reliable and repeatable measurements. As shown in the diagrams you canhave precision and accuracy independently, but you should aim to have themsimultaneously.