| Term |
Definition |
| 2-wire Resistance |
A standard method for measuring resistance by connecting two test leads across a resistance. |
| 4-wire Resistance |
A method for measuring resistance that provides better accuracy by eliminating the resistances of the test leads into the calculation. Usually, it consists of using four independent test leads to measure across a resistance, with one pair connected to a voltmeter and another to an ammeter. Special tools called Kelvin clips or probes are commonly used for this measurement to avoid contacts between the wires that may introduce stray resistance to the measuring circuit. |
| Accuracy |
States how closely an instrument indicates the actual value of a measured unit. Accuracy is usually indicated as an error percentage. Zero percent would indicate a perfect meter. Unless indicated, the accuracy percentage will be of full scale. |
| Averaging RMS |
The average value of a sine wave is 0.637 times the peak value, while the rms value of a sine wave is 0.707 times the peak value. This value varies depending on the type of wave being measured. For example, a triangle wave will have an rms value of 0.577 times the peak value. |
| CAT Rating |
The International Electrotechnical Commission (IEC) has defined voltage category ratings (CAT ratings) that indicate an instrument's ability to withstand voltage spikes. The usage for each CAT level is defined as:
- CAT I Signal level, telecomm electronics, special equipment
- CAT II Appliances, portable electronics
- CAT III Fixed installs, distribution level mains
- CAT IV Service drop to outside building |
| Common Mode Rejection Ratio |
A ratio of common mode voltage to input voltage, indicated as an error by the instrument. |
| Continuity Test |
A test usually accompanied by a buzzer sound to determine if the circuit under test has continuity, meaning its resistance is relatively low. |
| Counts |
A unit of measurement that defines the resolution a multimeter is capable of reading. The count value determines the limit point at which a multimeter will read before it loses resolution by one decimal point. For example, a 50,000 count multimeter will be able to read 49.999 volts, but at 50V, the multimeter may read 050.00 volts, losing one decimal place of resolution. |
| Crest Factor |
A ratio of signal peak to rms value. Defines the limits within which a true rms meter will accurately measure. |
| Data Hold |
A feature which, upon pressing a button holds (or freezes) the instantaneous reading on the display at the time the button is pressed. |
| dB |
A mathematical unit of measure, generally included as a feature in a multimeter for expressing DC and AC voltage, that minimizes the scope for a large range of measurements. It is often used to define the gain or signal-to-noise ratio. The formula to convert voltage to dB is:
dB = 20 log (Vin / Vref)
where Vin is the DC or AC voltage input and Vref is a reference voltage level. |
| dBm |
A mathematical unit of measure, often provided in a multimeter as a measurement tool to define a value in decibels above or below a 1mW reference. In general, 0dBm means the amount of voltage required to dissipate 1mW through a reference impedance is applied. The formula to calculate dBm is:
dBm = 10 log (((Vin)^2 / Zref) / 1mW )
where Vin is the DC or AC voltage input and Zref is the reference impedance defined by user. |
| Digits |
This is a common way that defines the resolution range, similar in respect to Counts. The values have specific meanings. For example, a 4 1/2 digits multimeter means it can read up to 4 digits from 0-9, while the 1/2 means it can read one digit from 0-1. Meaning, it can read 20,000 counts. This translates to reading within the range from 00000 to 19999. So the meter can read 19.999 volts, but at 20V or higher, it will lose a decimal point in resolution and read as 020.00 volts. |
| Frequency Response |
A band of frequencies over which the instrument will measure without exceeding a specified percentage of error. Usually but not always limited to a reference of 50/60Hz measurements. |
| Input Impedance |
The combined AC and DC resistance at the input of a Multimeter. In most cases you will see input impedances of 10 Mohms or greater for low DC voltage measurements. This virtually assures freedom from loading in most types of circuits. |
| LCD |
Is the abbreviation for Liquid Crystal Display. |
| LED |
Is the abbreviation for Light Emitting Diode. It is an electronic component that produces a bright light and has a long lifetime. |
| Normal Mode rejection Ratio |
A ratio of peak normal mode voltage to the equivalent pass band voltage, indicated as an error by the instrument. |
| Peak Hold |
A feature which holds the highest value attained during a measurement of changing levels of voltage or current. |
| PPM |
Abbreviated for Parts Per Million, literally meaning 1 PPM = 1/1,000,000. It is an alternate unit of measurement, often used in place of percentage unit for indicating resolution and accuracy. The following is a conversion list between PPM and Percent:
1 PPM = 0.0001%, 10 PPM = 0.001%, 100 PPM = 0.01%, 10000 PPM = 1%. |
| Resolution |
The smallest increment of change that can be indicated on a display. |
| RMS |
Is the abbreviation for Root Mean Square. It is the peak voltage of a sinusoidal waveform, times the square root of one half. |
| True RMS |
The true and accurate RMS measurement of a non-sinusoidal waveform. |
| VM Comp |
Often an option in a multimeter, it stands for voltmeter complete. It is generally a BNC terminal that provides a low-true pulse after each measurement has completed. In short, it is a complement with external trigger that is used as part of a handshake sequence between switching devices and measurement. |
