Keithley 2700, Руководство пользователя, Страница: 118 / 500

Model 2700 Multimeter/Switch System User’s Manual Basic DMM Operation 3-27
Ratiometric method
For the 10M
Ω
and 100M
Ω
ranges, the ratiometric method is used to measure resistance.
Test current for this method is generated by a 0.7µA current source (I
SOUR
) in parallel
with a 10M
Ω
reference resistance (R
REF
) as shown in Figure 3-11 .
Figure 3-11
Ratiometric method to measure ohms (10M
Ω
and 100M
Ω
ranges)
Basic circuit theory dictates that the sum of the branch currents (I
REF
and I
DUT
) is equal to
the source current (I
SOUR
). Since the voltmeter of the Model 2700 (V
MEAS
) has very high
input impedance (>10G
Ω
), current through the voltmeter branch is insignificant and can
be discounted. Therefore, as shown in Eq. 1, I
SOUR
= I
REF
+ I
DUT
.
2700
V
MEAS •
R
REF
R
DUT
(I
SOUR •
R
REF
) – V
MEAS
V
MEAS
R
REF
Eq. 1:
R
DUT
Input Hi
Input Lo
A) 2-wire ohms (
W2
) measurements (10M
W
and 100M
W
ranges)
B) 4-wire ohms (
W
4) measurements (10M
W
and 100M
W
ranges)
I
DUT
I
SOUR
= I
REF
+ I
DUT
= + V
MEAS
R
DUT
=
V
MEAS •
10M
W
(0.7µA
•
10M
W
) – V
MEAS
=
Eq. 2:
V
V
MEAS
I
SOUR
0.7uA
R
REF
10M W
I
REF
2700
R
DUT
Input Hi
Input Lo
I
DUT
V
V
MEAS
I
SOUR
0.7uA
R
REF
10M W
I
REF
Example:
Asume: V
MEAS
= 3.4V
3.4V
•
10M
W
(0.7µA
•
10M
W
) – 3.4V
=
= 9.444M
W
R
DUT
Sense Lo
Sense Hi
V
V
LEAD
R
LEAD
R
LEAD
W 2 Function
W 4 Function
V
DUT
= V
MEAS
- 2(V
LEAD
)
For Eq.1, Eq. 2 and the Example,
V
DUT
is used in place of V
MEAS
.