Voltage Measurements - Keithley 7710 User Manual

Table 1
Maximum test lead resistance (Model 2750)
Range R
1Ω
10Ω
Figure 2
Low-ohms measurements using 4-wire method
2750 DMM with 7710 card
V
V
Assumptions:
= V
R
DUT
V
V
SHI
V
SLO
V V
-
SHI SLO

Voltage measurements

As previously explained, path resistance can adversely affect
page 10). Series path resistance can also cause loading problems for DC voltage measurements
on the 100V, 10V, and 10mV ranges when the 10M Ω input divider is enabled.
High signal path resistance can also adversely affect AC voltage measurements on the 100V
range above 1kHz.
11
Ω Ω Ω Ω 2
R R
LEAD DUT LEAD
NA NA
0Ω 2Ω
Ch11 Hi
I
Ch1 Hi
TEST
V
SHI
I
M TEST
V
Ch1 Lo
SLO
Ch11 Lo
Virtually no current flows in the high-impedance sense circuit due to
the high impedance of the voltmeter (V
drops across Channel 11 and test leads 1 and 4 are negligible and
can be ignored.
The voltage drops across Channel 1 Hi (R
(R ) are not measured by the voltmeter (V
LEAD2
M
where: V is the voltage measured by the 2750.
M
I I is constant current sourced by the 2750 to the DUT.
TEST
TEST
=
V V
-
M
SHI SLO
× (R
I R
= +
TEST DUT LEAD3
× (R
I R
= +
TEST LEAD3
×
= I (R R
[ +
TEST DUT LEAD3
× R
= I
TEST DUT
= V
M
Ω Ω Ω Ω 4
R R
DUT LEAD
0Ω 1.2Ω
0Ω 7Ω
R
CH11Hi
Sense Hi
R
CH1Hi
Input Hi
R
CH1Lo Input Lo
R
Sense Lo
CH11Lo
). Therefore, the voltage
M
) and test lead 2
CH1Hi
).
M
R
+ )
CH1Lo
)
CH1Lo
R (R
+ ) - +
CH1Lo LEAD3
Ω Ω Ω Ω 4 Dry Circuit
R
DUT
Overflow Overflow
3Ω 12Ω
R
LEAD1
R
LEAD2
I
TEST
R
DUT
R
LEAD3
R
LEAD4
R
)]
CH1Lo
"Low-ohms measurements" (see