accuracy
PrintA measure of the capability of an instrument or sensor to faithfully indicate the value of the measured signal. This term is NOT related to resolution; however, the accuracy level can never be better than the resolution of the instrument. The accuracy of a voltmeter is often specified as:
Accuracy = error (in % of reading )+ offset
For example, a 5 ½ digit voltmeter can have an accuracy of 0.0125% of reading + 24 m V on its 2.5 V range, which results in an error of 149 m V when measuring a 1V signal. On the other hand, the resolution of this same voltmeter is 12 mV, or 12 times better than the accuracy.
It is important to note that the accuracy of an instrument depends not only on the instrument, but on the type of signal being measured.
The majority of the time, voltmeters are used to measure deterministic signals -- that is, signals that are stable DC voltages, or fixed sine waves.
However, if a signal also contains random noise, you must take into account the error formulae associated with the statistics. For example, measuring the RMS value of random noise may require significant averaging times before an accurate estimate can be made. If you measure the RMS value of a white noise signal with a 10 kHz bandwidth for 1 second, the statistical estimate will be accurate within 0.5%, a value which is often significantly "worse" than the basic "sine wave" accuracy of the instrument. There is nothing that you can do to the instrument to improve the accuracy, you can only improve it by increasing the averaging time of the measurement. In other words, since you are dealing with random data, you must have a significant number of data points to get a statistically accurate measurement. For a description of the error formulae and accuracy considerations for RMS measurements containing noise, see RMS.
For a description of the different types of signals you need to be aware of when deciding on your measurement technique, see the signal types.
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