Chapter 10 – EIS Measurement of Small Impedances--DC Errors and Four-terminal Measurements
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DC Errors and Four-terminal Measurements
Four-terminal measurements are a common technique used in precision measurement of small impedances. In
a four-terminal measurement, a nominally two-terminal device, such as a resistor or a battery, is connected
using four leads. Two of the leads carry the current that must flow through the device to make the
measurement. The other leads measure the voltage created by that current.
An illustration of two-terminal measurements versus four-terminal measurements can be seen in Figure 10-1.
Both schematic diagrams show a resistance measurement made by passing a known current through an
unknown resistance, R
test
. The wires in the circuit have a resistance R
wire
. A high input-impedance voltmeter
reads a voltage that is divided by the current value to calculate the value of R
test
.
In the two-terminal case, the voltage measurement is made using the same wires that carry the current. The
voltmeter measures:
V = I
test
x ( 2 H R
wire
+ R
test
)
The calculated R
test
is too high, since the resistance of the wires is added to the unknown resistance.
In the four-terminal case, the voltmeter uses two additional wires to measure the voltage close to R
test
. The
current is carried through the original pair of wires. There is no current through the voltmeter wires, so the wire
resistance does not create voltage drop. The voltmeter measures:
V = I
test
x R
test
The addition of two wires to the circuit eliminates the error caused by the resistance of the wires.
Figure 10-1
Two and Four Terminal Measurements
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