Section 8: Theory of operation
Series 3700A System Switch/Multimeter Reference Manual
8-8
3700AS-901-01 Rev. D/June 2018
Constant-current source method
For the 1
Ω to 1 MΩ ranges, the Series 3700A uses the constant-current method to measure
resistance. The Series 3700A sources a constant current (I
SOUR
) to the device under test (DUT) and
measures the voltage (V
MEAS
). Resistance (R
DUT
) is then calculated and displayed using the known
current and measured voltage.
Simplified schematics of the constant-current method are shown in the following figures. The test
current sourced to the DUT depends on the selected measurement range. For example, for the 100
Ω
range, the test current is 1 mA. Because the voltmeter of the Series 3700A has high input impedance
(>10
GΩ), virtually all the test current (1 mA) flows through the DUT. For a DUT that is
1
kΩ, 4-wire
ohms measurements should be used, as shown in the next figure. Because the voltage is measured
at the DUT, voltage drop in the test leads is eliminated (this voltage could be significant when
measuring a low-ohm DUT).
Figure 108: Two-wire constant-current source method
Figure 109: Four-wire constant-current source method
