Keithley 2700, User Manual, Page: 117 / 500

3-26 Basic DMM Operation Model 2700 Multimeter/Switch System User’s Manual
Constant-current source method
For the 100
Ω
to 1M
Ω
ranges, the Model 2700 uses the constant-current method to measure
resistance. The Model 2700 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 100
Ω
range the test
current is 1mA. Since the voltmeter of the Model 2700 has very high input impedance
(>10G
Ω
), virtually all the test current (1mA) flows through the DUT.
For DUT
≤
1k
Ω
, 4-wire ohms measurements should be used as shown in Figure 3-10 B. The
voltage is measured at the DUT. This eliminates IR drop in the test leads, which could be
significant when measuring low ohm DUT.
Figure 3-10
Constant-current method to measure ohms (100
Ω
to 1M
Ω
ranges)
DUT
V
I
SOUR
V
MEAS
Input Hi
Input Lo
2700
DUT
V
I
SOUR
V
MEAS
Input Hi
Input Lo
2700
Sense Hi
Sense Lo
A) 2-wire ohms (
W
2) measurements (100
W
through 1M
W
ranges)
R
DUT
= V
MEAS
I
SOUR
R
DUT
= V
MEAS
I
SOUR
B) 4-wire ohms (
W
4) measurements (100
W
through 1M
W
ranges)