Keithley 237 Instruction Manual

Operation
4-8
Voltage divider checks
For thick Þlm resistor networks that are to be used as
voltage dividers, it may be desirable to test them using
voltages that simulate actual operating conditions.
This is a particularly useful test for resistor networks
that have a voltage coefÞcient speciÞcation. The test
system in Figure 4-4 uses a Keithley Model 230 to
source voltage and the Model 196 to measure voltage.
A consideration in these checks is the effect of the Mod-
el 196 input impedance on voltage measurements. The
input impedance is shunted across the resistor being
measured. The resultant divider resistance is the paral-
lel 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 intro-
duced into the measurement will be minimal. Mini-
mum input impedance requirements are, of course,
determined by the accuracy needed in the measure-
ment. The input impedances of the Model 196 are as
follows: 300mV and 3V ranges, 1G Ω
; 30V range, 11M Ω ;
300V range, 10.1M Ω . For better input impedance re-
quirements, the Keithley Model 617 Electrometer can
be incorporated into the test system to measure volt-
age.
Another factor to be considered when checking low
voltage dividers is thermal EMFs generated by the ma-
trix card. (The Model 7012 has been designed to keep
relay EMF at a minimal level.) A matrix card crosspoint
can generate up to 5 µ V 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 196 and the resistor networks, the sense leads
are internally disconnected from the input of the DMM
when the volts function is selected. The simpliÞed test
system is shown in Figure 4-6.
The thick Þlm is tested by applying a voltage across the
resistor network and measuring the voltage across
each resistor element and/or across combined ele-
ments. In Figure 4-6, crosspoints 1!3!1 and 1!4!4 are
closed to apply voltage across the network, and cross-
points 1!1!3 and 1!2!4 are closed to measure the voltage
drop across R3.
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Keithley 237 Specifications

General

Brand	Keithley
Model	237
Category	Measuring Instruments
Language	English