Keithley 2701 - Page 133

Model 2701 User’s Manual Basic DMM Operation 3-29
Figure 3-11
Ratiometric method to measure ohms (10MΩ and 100MΩ ranges)
2701
VMEAS • RREF
RDUT
(ISOUR • RREF) – VMEAS
VMEAS
RREF
Eq. 1:
RDUT
Input Hi
Input Lo
A) 2-wire ohms (Ω2) measurements (10MΩ and 100MΩ
ranges)
B) 4-wire ohms (
Ω4) measurements (10MΩ and 100MΩ
ranges)
IDUT
ISOUR = IREF + IDUT
=
+
V
MEAS
RDUT
=
V
MEAS • 10MΩ
(0.7µA • 10MΩ) – VMEAS
=
Eq. 2:
V
VMEAS
ISOUR
0.7uA
RREF
10MΩ
IREF
2701
RDUT
Input Hi
Input Lo
IDUT
V
VMEAS
ISOUR
0.7uA
RREF
10MΩ
IREF
Example:
Asume: V
MEAS = 3.4V
3.4V
• 10MΩ
(0.7µA • 10MΩ) – 3.4V
=
= 9.444M
Ω
RDUT
Sense Lo
Sense Hi
V
VLEAD
RLEAD
RLEAD
Ω 2 Function
Ω 4 Function
VDUT = VMEAS - 2(VLEAD)
For Eq.1, Eq. 2 and the Example,
V
DUT is used in place of VMEAS.