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3.6 Optional Scanner Installation 63
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3.6 Optional
Scanner
Installation The 3726 and 3708 scanners expand the input capability of the Model 372 and
increase the number of input channels to 16 (3726) or 8 (3708). It is an extension of
the isolated measurement circuitry of the Model 372 so its installation should follow
the guidelines described in section 3.5 as well as this paragraph. Cables between the
scanner and Dewar should be as short as possible because they carry the smallest sig-
nals. Cables between the scanner and instruments can be longer.
3.6.1 Grounding the
Scanner The enclosures of the 3726 and 3708 scanners are connected to the input cable
shield, and therefore, are tied to measurement common (not the chassis) of the
Model 372. In most installations the shield screw on the enclosure should be ground
strapped to the test Dewar creating the single connection between measurement cir-
cuits and the earth. If a different shield to earth connection is preferred, the scanner
enclosure must be electrically insulated from its surroundings.
3.6.2 Scanner Resistor
Input Connector
Definition With the 3726 scanner, four 25-pin D-sub-miniature receptacles hold connections
for up to sixteen 4-lead resistor inputs in four sets of four. With the 3708 scanner, two
25-pin DIN receptacles hold connections for up to eight 4-lead resistor inputs. They
mate with 25-pin D-sub-miniature plugs included with the scanner (Lake Shore P/N
106-253). Unused inputs can either be left open or all four measurement pins can be
shorted together. Pin connections for channels 1 through 4 are shown in FIGURE 3-9,
channels 5 through 8, 9 through 12, and 13 through 16 are wired the same way.
3.6.3 Scanner Resistor
Input Cabling The cable between scanner and Dewar should be kept as short as possible without
causing strain on the cable or connections (FIGURE 3-7). Short cables can improve the
measurement in several ways. They minimize the potential for induced noise on the
smallest voltage signals, limit cable capacitance that cannot be guarded because the
guards are not carried through the scanner, and reduce the potential for ground loops
if shield lines accidentally touch the Dewar.
Individually shielded, twisted pair cable is recommended for wiring the resistor
inputs. Cable containing up to eight shielded pairs of wire is available from electron-
ics distributors so one cable can provide enough wires to connect the four channels of
one input connector. If multiple smaller cables are used they should be wrapped
together to minimize ground loops. The two current leads of a measurement input
channel should be twisted and the two voltage leads should be twisted. The input
connectors provide several shield pins so that individual shields can be connected
separately at the scanner end of the cable.
If the shield-screw on the scanner enclosure is strapped the test Dewar, the cable
shield can be left unterminated at the Dewar end. If the scanner enclosure is not con-
nected to the Dewar, the shields of the cable should be terminated at the bulk head
connection or other conductive point on the Dewar.
FIGURE 3-8 3726 scanner front and rear panels