Noise - Keithley 6521 Instruction Manual

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Figure 3-2
Voltage attenuation by path isolation resistance

3.6.3 Noise

Noise induced into a switching system can have serious det-
rimental effects on measurements made with the system.
Chief among noise considerations are magnetic fields, radio
frequency interference, and ground loops.
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
scanning system. If the conductor has sufficient length, even
weak magnetic fields like those of the earth can create suffi
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 flux densities (such as mu met-
al) is effective at reducing these effects.
Even when the conductor is stationary, magnetically-in-
duced 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.
Electromagnetic interference (EMI)
The electromagnetic interference characteristics of the Mod-
el 6517 Electrometer/High Resistance System comply with
the electromagnetic compatibility (EMC) requirements of
the European Union (EU) directives as denoted by the CE
mark. However, it is still possible for sensitive measurements
to be affected by external sources. In these instances, special
precautions may be required in the test setup.
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Sources of EMI include:
• Radio and TV broadcast transmitters.
• Communications transmitters,
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 instrument, measurement leads, and other cables should
be kept as far away as possible from any EMI sources.
Shielding measurement leads and the multimeter may reduce
EMI to acceptable levels. In extreme cases, a specially con-
structed screen room may be required to sufficiently attenu-
ate troublesome signals.
Many instruments incorporate internal filtering that may
help reduce EMI effects. In some cases, additional external
filtering may be required. Keep in mind, however, that filte -
ing may have detrimental effects on the measurement.
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 3-3, the resulting ground loop causes
current to fl w 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.
Signal Leads
Instrument 1 Instrument 2
Ground Loop
Current
Figure 3-3
Power line ground loops
Figure 3-4 shows how to connect several instruments togeth-
er to eliminate this type of ground loop problem. Here, only
one instrument is connected to power line ground. Ground
loops are not normally a problem with instruments having
Operation
including cellular
Instrument 3
Power Line Ground
3-9