Keithley 2701

3-26 Basic DMM Operation Model 2701 User’s Manual
Constant-current source method
For the 100Ω to 1MΩ ranges, the Model 2701 uses the constant-current method to
measure resistance. The Model 2701 sources a constant current (I
SOUR
) to the DUT and
measures the voltage (V
MEAS
). Resistance (R
DUT
) is then calculated (and displayed)
using the known current and measured voltage (R
DUT
= V
MEAS
/I
SOUR
).
The constant-current method is shown in Figure 3-10. The test current sourced to the DUT
depends on the selected measurement range. For example, for the 100Ω range, the test
current is 1mA. Since the voltmeter of the Model 2701 has very high input impedance
(>10GΩ), virtually all the test current (1mA) flows through the DUT.
For DUT ≤1kΩ, 4-wire ohms measurements should be used as shown in Figure 3-10B.
The voltage is measured at the DUT. This eliminates IR drop in the test leads, which could
be significant when measuring low ohm DUT.