Keithley 7037, Инструкция по эксплуатации

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Operation
5-8
Measurement considerations
Many measurements made with the Model 7037 are subject
to various effects that can seriously affect low-level measure-
ment accuracy. The following paragraphs discuss these
effects and ways to minimize them.
Path isolation
The path isolation is simply the equivalent impedance
between any two test paths in a measurement system. Ideally,
the path isolation should be infinite, but the actual resistance
and distributed capacitance of cables and connectors results
in less than infinite path isolation alues for these devices.
Path isolation resistance forms a signal path that is in parallel
with the equivalent resistance of the DUT, as shown in Fig-
ure 5-7. For low-to-medium device resistance values, path
isolation resistance is seldom a consideration; however, it
can seriously degrade measurement accuracy when testing
high-impedance devices. The voltage measured across such
a device, for example, can be substantially attenuated by the
voltage divider action of the device source resistance and
path isolation resistance, as shown in Figure 5-8. Also, leak-
age currents can be generated through these resistances by
voltage sources in the system.
Figure 5-7
Path isolation resistance
Figure 5-8
Voltage attenuation by path isolation resistance
Any differential isolation capacitance affects DC measure-
ment settling time as well as AC measurement accuracy.
Thus, it is often important that such capacitance be kept as
low as possible. Although the distributed capacitance of the
card is generally fi ed by design, there is one area where you
do have control over the capacitance in your system: the con-
necting cables. To minimize capacitance, keep all cables as
short as possible.
Magnetic fields
When a conductor cuts through magnetic lines of force, a
very small current is generated. This phenomenon will fre-
quently cause unwanted signals to occur in the test leads of a
relay switching system. If the conductorhas sufficient length,
even weak magnetic fields like those of the earth can create
sufficient signals to a fect low-level measurements.
Two ways to reduce these effects are: (1) reduce the lengths
of the test leads, and (2) minimize the exposed circuit area.
In extreme cases, magnetic shielding may be required. Spe-
cial metal with high permeability at low flux densities (such
as mu metal) is effective at reducing these effects.
Even when the conductor is stationary, magnetically induced
signals may still be a problem. Fields can be produced by
various signals such as the AC power line voltage. Large
inductors such as power transformers can generate substan-
tial magnetic fields, so care must be taken to keep the switch-
ing and measuring circuits a good distance away from these
potential noise sources.
R
E
DUT
DUT
R
PATH
V R
IN
DUT
7037
Card
Measure
Instrument
= Source Resistance of DUT
= Source EMF of DUT
= Path Isolation Resistance
= Input Resistance of Measuring Instrument
R
DUT
E
DUT
R
PATH
R
IN
R
E
DUT
DUT
R
PATH
E
OUT
R
PATH
R
DUT
R
PATH
+
=
E
DUT