Sensor Lead Cable (Continued)
The arrangement of wires in a cable is also important. For best results, twist
voltage leads, V+ and V– together and twist current leads I+ and I–
together. Cover the twisted pairs of voltage and current leads with a braided
or foil shield connected to the shield pin of the instrument. This type of
cable is available through local electronics suppliers. Instrument
specifications are given assuming 10 feet of sensor cable. Longer cables,
100 feet or more, can be used but environmental conditions may degrade
accuracy and noise specifications.
2.6.3 Shielding Sensor Leads
Shielding the sensor lead cable is important to keep external noise from
entering the measurement. The sensor lead cable should be shielded
whenever possible. In many systems, it is impractical to shield the sensor
leads inside the cryostat. In theses cases, the cable shield should still be
used on the room temperature sensor leads up to the cryostat.
A shield is most effective when it is near the measurement potential, so the
Model 211 offers a shield pin on the Input/Output Connector that stays
close to the measurement. The shield pin is tied to chassis ground and
should be used as the connection point for the sensor cable shield.
Depending on how the instrument is grounded, the shield may or may not
need to be terminated at the opposite end. See Paragraph 2.6.4 below on
instrument grounding.
2.6.4 Instrument Grounding
The Model 211 does not provide isolation between measurement circuits
and chassis ground. The measurement leads have a finite impedance to
chassis ground and should not be tied to ground outside the instrument or an
error in reading may result. The Model 211 has the best noise performance
when the chassis is tied to earth ground. This connection should be made at
only one point so as to avoid ground loops.
If the sensor leads are shielded, the cable shield should be tied to the shield
pins on the Input/Output connector but should not be terminated at the other
end.
The standard power supply (109-132) does not connect the common pins to
earth ground. The connection should be made externally. If the sensor leads
are shielded, one end of the cable shield can be tied to the cryostat ground
while the other end is tied to the shield pins on the Input/Output connector.
If the sensor leads are not shielded, the instrument chassis should be
strapped to earth ground.
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