Idiomas disponibles
Idiomas disponibles
4 Measurement Tutorials
Resistance measurement
2-wire resistance measurements only supported on the DAQM900A, DAQM901A, DAQM902A, and
DAQM908A multiplexer modules.
4-wire resistance measurements only supported on the DAQM900A, DAQM901A, and DAQM902A multiplexer
modules.
An ohmmeter measures the DC resistance of a device or circuit connected to its input. Resistance measurements
are performed by supplying a known DC current to an unknown resistance and measuring the DC voltage drop.
The internal DMM offers two methods for measuring resistance: 2-wire and 4-wire ohms. For both methods, the test
current flows from the input HI terminal through the resistor being measured. For 2-wire ohms, the voltage drop
across the resistor being measured is sensed internal to the DMM. Therefore, test lead resistance is also measured.
For 4-wire ohms, separate "sense" connections are required. Since no current flows in the sense leads, the
resistance in these leads does not give a measurement error.
4-Wire Ohms measurements
The 4-wire ohms method provides the most accurate way to measure small resistances. Test lead, multiplexer, and
contact resistances are automatically reduced using this method. The 4-wire ohms method is often used in
automated test applications where long cable lengths, input connections, and a multiplexer exist between the
internal DMM and the device-under-test.
The recommended connections for 4-wire ohms measurements are shown in the diagram on the following page. A
constant current source, forcing current I through unknown resistance R, develops a voltage measured by a DC
voltage front end. The unknown resistance is then calculated using Ohm's Law.
The 4-wire ohms method is used in systems where lead resistances can become quite large and variable and in
automated test applications where cable lengths can be quite long. The 4-wire ohms method has the obvious
disadvantage of requiring twice as many switches and twice as many wires as the 2-wire method. The 4-wire ohms
method is used almost exclusively for measuring lower resistance values in any application, especially for values less
than 10 Ω and for high-accuracy requirements such as RTD temperature transducers.
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Keysight DAQ970A/DAQ973A User's Guide
