Keithley 1801, Инструкция по эксплуатации, Страница: 45 / 93

Operation
3-22
3.4.4 Electromagnetic interference (EMI)
The electromagnetic interference characteristics of the
Model 1801 Nanovolt Preamp comply with the electro-
magnetic compatibility (EMC) requirements of the
European Union (EU) directives as denoted by the CE
mark. However, it is still possible for sensitive measure-
ments to be affected by external sources. In these
instances, special precautions may be required in the test
setup.
Sources of EMI include:
• Radio and TV broadcast transmitters.
• Communications transmitters, including cellular
phones and handheld radios.
• Devices incorporating microprocessors and high-
speed digital circuits.
• Impulse sources as in the case of arcing in high-
voltage environments.
The Model 1801, signal source, and signal leads should be
kept as far away as possible from any EMI sources. Addi-
tional shielding of the instrument, signal leads, sources,
and other measuring instruments will often reduce EMI to
an acceptable level. In extreme cases, a specially con-
structed screen room may be required to sufficiently atten-
uate the troublesome signal.
The Model 1801 filters may help to reduce EMI effects in
some situations. In other cases, additional external filter-
ing may be required. Keep in mind, however, that filtering
may have detrimental effects, such as increased settling
time, on the measurement.
3.4.5 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 instrumentation
with more than one signal return path such as power line
ground. As shown in Figure 3-10, the resulting ground
loop causes current to flow 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 terminals of the two instruments.
This voltage will be added to the source voltage, affecting
the accuracy of the measurement.
Figure 3-11 shows how to connect instruments together to
eliminate this type of ground loop problem. Here, the cir-
cuit is grounded only at one point in the system. Although
some common-mode current still flows through the com-
mon-mode impedance Z
CM
, that current is very small
(typically nA or less), and the generated error voltage is
insignificant compared to the source voltage.
Ground loops are not normally a problem with instru-
ments like the Model 1801 that have low common-mode
current. However, all instruments in the test setup may not
be designed in this manner. When in doubt, consult the
manual for all instrumentation in the test setup.
3.4.6 Shielding
Proper shielding of all signal paths and sources being
measured is important to minimize noise pickup in virtu-
ally any low-level measurement situation. Otherwise,
interference from such noise sources as line frequency
and RF fields can seriously corrupt measurements, com-
promising the validity of experimental data.
In order to minimize noise, a closed metal shield, com-
pletely surrounding the source, is recommended. This
shield should be connected to input LO, and LO may also
have to be connected to chassis ground. Some experimen-
tation may be required to determine which of the two con-
necting methods provides the best results.