Sensor Lead Cable; Grounding And Shielding Sensor Leads; Sensor Polarity - Lakeshore 336 User Manual

34 c 3: Installation
HAPTER

3.5.2 Sensor Lead Cable

3.5.3 Grounding and
Shielding Sensor Leads

3.5.4 Sensor Polarity

Model 336 Temperature Controller
The sensor lead cable used outside the cooling system can be much different from
what is used inside. Between the instrument and vacuum shroud, heat leak is not a
concern. In this case, choose cabling to minimize error and noise pick up. Larger con-
ductor, 22 AWG to 28 AWG stranded copper wire is recommended because it has low
resistance yet remains flexible when several wires are bundled in a cable. The
arrangement of wires in a cable is also important. For best results, voltage leads, V+
and V- should be twisted together and current leads I+ and I- should be twisted
together. The twisted pairs of voltage and current leads should then be covered with a
braided or foil shield that is connected to the shield pin of the instrument. This type of
cable is available through local electronics suppliers. Instrument specifications are
given assuming 3 m (10 ft) of sensor cable. Longer cables, 30 m (100 ft) or more, can
be used, but environmental conditions may degrade accuracy and noise specifica-
tions. Refer to section 2.4.6 for information about wiring inside the cryostat.
The sensor inputs are isolated from earth ground to reduce the amount of earth
ground referenced noise that is present on the measurement leads. Connecting sen-
sor leads to earth ground on the chassis of the instrument or in the cooling system
will defeat that isolation. Grounding leads on more than one sensor prevents the sen-
sor excitation current sources from operating.
Shielding the sensor lead cable is important to keep external noise from entering the
measurement. A shield is most effective when it is near the measurement potential so
the Model 336 offers a shield at measurement common. The shield of the sensor
cable should be connected to the shield pin of the input connector. The shields should
not be connected to earth ground on the instrument chassis. One shield should be
connected to the cryostat's ground as long as it is near earth ground. Connecting at
more than one point will cause a ground loop, which adds noise to the measurement.
The shells of the input connectors are at the same potential as the shield pin on the
Model 336. Older Lake Shore controllers are not configured this way.
This section describes the diode/resistor sensor inputs.
Lake Shore sensors are shipped with instructions that indicate which sensor leads are
which. It is important to follow these instructions for plus and minus leads (polarity)
as well as voltage and current when applicable. Diode sensors do not operate in the
wrong polarity. They look like an open circuit to the instrument. Two-lead resistors
can operate with any lead arrangement and the sensor instructions may not specify.
Four-lead resistors can be more dependent on lead arrangement. Follow any speci-
fied lead assignment for four-lead resistors. Mixing leads could give a reading that
appears correct but is not the most accurate.
Cathode
FIGURE 3-4 DT-670-SD Diode sensor leads
Anode