Keithley 7022, Instruction Manual

Page: 67 / 107

Operation
5-13
Figure 5-12
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 fixed 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
switching matrix system. If the conductor has sufficient
length, even weak magnetic fields like those of the earth can
create sufficient signals to affect 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.
At high current levels, even a single conductor can generate
significant fields. These effects can be minimized by using
twisted pairs, which will cancel out most of the resulting
fields.
Radio frequency interference
Radio Frequency Interference (RFI) is a general term used to
describe electromagnetic interference over a wide range of
frequencies across the spectrum. Such RFI can be particu-
larly troublesome at low signal levels, but it can also affect
measurements at high levels if the problem is of sufficient
severity.
RFI can be caused by steady-state sources such as radio or
TV signals or some types of electronic equipment (micropro-
cessors, high speed digital circuits, etc.), or it can result from
impulse sources, as in the case of arcing in high-voltage envi-
ronments. In either case, the effect on the measurement can
be considerable if enough of the unwanted signal is present.
RFI can be minimized in several ways. The most obvious
method is to keep the equipment and signal leads as far away
from the RFI source as possible. Shielding the switching
card, signal leads, sources, and measuring instruments will
often reduce RFI to an acceptable level. In extreme cases, a
specially constructed screen room may be required to suffi-
ciently attenuate the troublesome signal.
Many instruments incorporate internal filtering that may
help to reduce RFI effects in some situations. In some cases,
additional external filtering may also be required. Keep in
mind, however, that filtering may have detrimental effects on
the desired signal.
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