Lakeshore 336, User Manual

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3.6.1 Wiring, Guarding and Shielding 37
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3.6.1 Wiring, Guarding
and Shielding The capacitance input uses the same 6-pin din connector as the standard inputs, and
the same pins for current excitation and voltage feedback. Cable capacitance in lon-
ger cables can cause large sensor reading errors if proper guarding and shielding
methods are not applied. To address this problem, a driven guard is provided on pin 6,
and a shield pin is provided on pin 3. The guard pin should be connected to a foil shield
that surrounds a single twisted pair of wires used for I+ and V+. The shield pin on pin 3
should be connected to a foil shield that surrounds a single twisted pair of wires used
for I- and V-. See FIGURE 3-7. This wiring scheme must be applied to ensure proper
sensor readings using the Model 3061 capacitance option
The 3.496 kHz excitation of the option card can interfere with the sensitive DC measure-
ments of the standard inputs. Tightly twist the lead wires of each sensor and separate
them from the leads from the other sensor. Test any system for sensor interference before
it is permanently sealed
3.7 Thermocouple
Sensor Inputs
(Thermocouple
Model 3060)
The information in this section is for a Model 336 configured with thermocouple sen-
sor inputs. Thermocouple inputs are not installed on the standard Model 336, but can
be added by purchasing the Model 3060 dual thermocouple input option. Refer to
section 7.6 for installation of the Model 3060.
Do not leave thermocouple inputs unconnected. Short inputs when not in use.
3.7.1 Sensor Input
Terminals Attach sensor leads to the screws on the off-white ceramic terminal blocks. Sensor
connection is important when using thermocouples because the measured signal is
small. Many measurement errors can be avoided with proper sensor installation. The
block has two thermocouple inputs and each input has two screw terminals; one pos-
itive, one negative. See FIGURE 3-8.
FIGURE 3-7 Capacitance Input shield and gard
+
+
–
–
V
I
V
I
Cs
Shield
Guard