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Digital Multimeter Basics


A Digital Multimeter (DMM) is used to read voltage, current, and resistance. A knob or button is used to select the function (volts, amps, ohms) and the range (2, 20, 200 etc.). The selector often has DC VOLTS, AC VOLTS, AC mA, DC mA, OHMS, and TEMP settings. Within each of these settings is the range. For instance, on the DC VOLTS setting, there is 200mV, 2V, 20V, 200V, and 1K (1000V). These are the ranges. The range determines the maximum reading the DMM can display. If the reading goes above that value, the display flashes and the reading is meaningless.

The display is an LED readout that shows what the measurement is. The display often has 3 full digits of resolution, plus a 4th digit (on the left) that can be a 1 or blank. This is called a 3 1/2 digit display. The decimal point is inserted depending on what range the DMM is set to. This allows the meter to read from 0.000V to 1.999V on the 2V range, or from 0.0mV to 199.9mV on the 200mV range. The last digit on the right determines the minimum value that can be distinguished. This is called the resolution of the meter. For instance, on the 2V range, the last, or least significant, digit is the 1 millivolt digit. So the resolution is 1mV and the meter can distinguish 1.495V from 1.496V. On the 20V scale, however, the least significant digit (and thus the resolution) is the 10mV place. So on the 20V scale, the meter cannot distinguish between 1.495V and 1.496V. The meter reads 1.49V or 1.50V instead. For this reason (and others), you should usually make your readings on the lowest range possible. The resolution is determined the same way for current, resistance, and voltage.

In addition to the resolution of the meter, there are two other important parameters: internal resistance and accuracy.


A voltmeter is connected in parallel with the element you are testing, while an ammeter is connected in series. This is shown below.

Ideally, a voltmeter should have infinite parallel resistance and an ammeter should have zero series resistance. The series resistance for the current scales depends on the range. The DC ammeter resistance is found using the formula

R = (0.2V / Range Setting) + 0.1Ω

If the meter is set to read 20mA full scale, the resistance of the meter would be

R = (0.2V / 20mA) + 0.1Ω = 10Ω + 0.1Ω = 10.1Ω

The meter resistance is an important parameter and can affect the reading. While it can be neglected at times, a good engineer will take it into consideration before making the measurement.


Accuracy is often confused with resolution. It is possible to have a meter that will read values to microvolt resolution, but only have millivolt accuracy. This is because accuracy is determined by the components used inside, and not just the display. The DM502 has a formula for determining the meter accuracy (excluding any errors due to the resistance mentioned above).

On DC VOLTS, the accuracy for the 20V scale is 0.1%, plus or minus 1 count, where 1 count means the least significant digit or resolution for the range it is set to. As an example, let us take a reading of 5.23V on the 20V range. The resolution on the 20V range is 0.01V (10mV). The accuracy is

0.1%(5.23V) + 10mV = 5.23nV + 10mV = 15.23mV

This is an overall error of

%Error = (15.23mV / 5.23V) • 100% = 0.291%

A reading of 1.67V on the 20V scale yields

0.1%(1.67V) + 10mV = 1.67mV + 10mV = 11.67mV or 0.7%

It is interesting to note that on the 2V scale, a measurement of 1.67V gives an error of only 0.16%. So, not only do we get better resolution on a lower range, we get better accuracy. This is usually the case. Typically, your measurements will be limited by the 5% accuracy of resistors used and not the accuracy of the meter. But it is a good idea to check, just in case.

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