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Operation
5-9
A consideration in these checks is the effect of the Model
2000 input impedance on voltage measurements. The input
impedance is shunted across the resistor being measured.
The resultant divider resistance is the parallel combination of
the resistor under test and the input impedance. As long as
the input impedance is much larger than the resistor being
tested, the error introduced into the measurement will be
minimal. Minimum input impedance requirements are deter-
mined by the accuracy needed in the measurement. The input
impedances of the Model 2000 are as follows: 10mV, 1V, and
10V ranges, 10G
Ω
; 100V and 1,000V ranges, 10M
Ω
. For
better input impedance requirements, the Keithley Model
6517A Electrometer can be incorporated into the test system
to measure voltage.
Another factor to be considered when checking low voltage
dividers is thermal EMFs generated by the card. (The Model
7022 has been designed to keep relay EMF at a minimal
level.) A matrix crosspoint can generate up to 3µV of thermal
EMF. Thus, when making low voltage measurements, be
sure to account for this additional error.
Even though four-terminal connections are made at the
Model 2000 and the resistor networks, the sense leads are
internally disconnected from the input of the DMM when the
volts function is selected. The simplified test system is
shown in Figure 5-7.
HI
LO
POWER
Model 2000
Input HI
Input LO
Output
Common
Model 230
Measure V
Source V
H
A
B
Model 7022
C
V
2000
230
+/-
Equivalent Circuit
D
L
Sense Output
Sense Common
R1
R 2
R 3
Thick Film
H
L
H
L
H
L
H
L
H
L
H
L
H
L
R1
R 2
R 3
H
H
H
H
1
2
3
4
Rows
H
L
5
R 4
X
X
X
X
X = Closed Crosspoints
R 4
H
6
Cols
E
H
L
Figure 5-7
Voltage divider checks
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