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10.6 MODEL 3468 EIGHT-CHANNEL INPUT OPTION CARD
10.6.1
General
The 3468 option adds 8 sensor inputs to the Model 340 Temperature Controller. The original standard inputs
remain fully functional allowing the Model 340 to measure 10 sensors when the option is installed. The
optional inputs are broken into two groups of four and appear on the display as C1—C4 for Input C, D1—D4
for Input D. The option card includes two A/D converters, one for each group of four inputs, and individual
excitation for each sensor. Each group of inputs must use the same sensor type, but the two groups can be
different. Calibration for the option is stored on the card so it can be installed in the field without recalibration.
The 3468 option supports all the sensors as the Model 340 standard inputs. Diode and Platinum
configurations have similar specifications to the standard inputs, reduced only slightly to account for
multiplexing. However, the NTC RTD configuration is quite different than the standard inputs. The option has
a limited resistance range of 7.5 kΩ with a fixed current excitation of 10 µA, compared to the 300 kΩ
resistance range and voltage excitation available on the standard inputs. This limitation significantly reduces
the low temperature range of the inputs. The option also does not support current reversal to reduce the effect
of thermal EMF voltages.
10.6.2
Option Installation
The 3468 can be installed into a Model 340 at the factory or in the field. To field install a 3468 option follow
the installation procedure in Paragraph 10.3.1. Only one option board can be installed in the Model 340 at a
time. To verify which option, if any, is installed in a Model 340 press Options, and then press More three
times. Option identification displays on the REVISION INFORMATION screen. If options are changed, the
Model 340 changes all parameter settings to default.
10.6.3
Sensor Installation
10.6.3.1 Sensor Input Connector
Two 25 pin D subminiature receptacles hold connections for the C1-C4 and D1-D4 sensor inputs. They mate
with 25 pin D subminiature plugs (Lake Shore P/N 106-253) included with the option. Additional mating
connectors can either be ordered from Lake Shore or purchased from local electronics distributors. Figure
10-10 illustrates pin assignments and pin numbering as seen from the rear of the option card.
It is possible for an overload condition on one channel to affect the reading on another in the same input
group so unused sensor inputs should have all four connections shorted together. Wire redundant sensors in
separate groups for best reliability. Split fewer than eight sensors evenly between input groups for best
reading efficiency.
10.6.3.2 Sensor Input Cabling
Twisted shielded cable is recommended for wiring to the 3468 outside of the cryostat. Cable shields should
be connected to the shield (S) pins of the input connector. There are enough shield pins in to connect an
individual shield for each input if necessary. The body of the connector is at earth ground and not the options
measurement common; therefore it does not make an effective shield. The connector body should not be
connected to any signal or shield lines during measurement because it can cause ground loop noise.
10.6.3.3 Reading Rate
Both inputs update every 400 milliseconds when fully loaded with sensors. Update rate improves with fewer
active sensors in a group of four as shown below. To disable unused sensor inputs use the Input Setup
screen (Paragraph 10.6.4.2).
# of enabled channels
10-14
Lake Shore Model 340 Temperature Controller User's Manual
Update rate
1
2
200 ms
3
300 ms
4
400 ms
# of readings / second
50 ms
20
5
3.3
2.5
Options and Accessories
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