Keithley 7012-S, Инструкция по эксплуатации, Страница: 60 / 93

Operation
4-14
Any differential isolation capacitance affects DC mea-
surement settling time as well as AC measurement ac-
curacy. Thus, it is often important that such capacitance
be kept as low as possible. Although the distributed ca-
pacitance of the matrix card is generally
Þ xed by de-
sign, there is one area where you do have control over
the capacitance in your system; the connecting cables.
To minimize capacitance, keep all cables as short as
possible.
4.5.2 Magnetic fields
When a conductor cuts through magnetic lines of force,
a very small current is generated. This phenomenon
will frequently cause unwanted signals to occur in the
test leads of a switching matrix system. If the conduc-
tor has suf
Þ cient length, even weak magnetic Þ elds like
those of the earth can create suf Þ cient 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. Special metal with high permeability at
low
ß ux densities (such as mu metal) is effective at re-
ducing 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 substantial magnetic
Þ elds, so care must
be taken to keep the switching and measuring circuits
a good distance away from these potential noise sourc-
es.
At high current levels, even a single conductor can gen-
erate signi
Þ cant Þ elds. These effects can be minimized
by using twisted pairs, which will cancel out most of
the resulting
Þ elds.
4.5.3 Radio frequency interference
RFI (Radio Frequency Interference) is a general term
used to describe electromagnetic interference over a
wide range of frequencies across the spectrum. Such
RFI can be particularly troublesome at low signal lev-
els, but is can also affect measurements at high levels if
the problem is of suf
Þ cient severity.
RFI can be caused by steady-state sources such as radio
or TV signals, or some types of electronic equipment
(microprocessors, high speed digital circuits, etc.), or it
can result from impulse sources, as in the case of arcing
in high-voltage environments. 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 obvi-
ous method is to keep the equipment and signal leads
as far away from the RFI source as possible. Shielding
the matrix switching card, signal leads, sources, and
measuring instruments will often reduce RFI to an ac-
ceptable level. In extreme cases, a specially-construct-
ed screen room may be required to suf
Þ ciently
attenuate the troublesome signal.
Many instruments incorporate internal Þ ltering that
may help to reduce RFI effects in some situations. In
some cases, additional external
Þ ltering may also be re-
quired. Keep in mind, however, that Þ ltering may have
detrimental effects on the desired signal.
4.5.4 Ground loops
When two or more instruments are connected together,
care must be taken to avoid unwanted signals caused
by ground loops. Ground loops usually occur when
sensitive instrumentation is connected to other instru-
mentation with more than one signal return path such
as power line ground. As shown in Figure 4-12, the re-
sulting ground loop causes current to
ß ow through the
instrument LO signal leads and then back through
power line ground. This circulating current develops a
small but undesirable voltage between the LO termi-
nals of the two instruments. This voltage will be added
to the source voltage, affecting the accuracy of the mea-
surement.
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