Model 2790 SourceMeter
®
Switch System User’s Manual 4-9
Figure 4-6
Installed Zippertubing
®
shield
Guarding
NOTE The typical high ohms measurement applications for the 7751/7753 V-source
does not require guarding. Guarding only needs to be used for the most demand-
ing, noisy situations, or when measuring >1GΩ (which is the maximum resis-
tance specified for the 7751/7753).
A second inner shield can be used for high ohms measurements that use the 7751/7753
V-source and I/V converter. This guarded shield may reduce noise and cable leakage that
could result in measurement errors.
The guard terminals are available at J105 and J107. Guard is connected to a shield that
surrounds the Source Lo and Sense Lo test leads that connect to the housing of the inflator.
In Figure 5-3 (Section 5), use the guard shield to surround the two wires that are con-
nected to Bank 3 (J103) of the 7751 module.
This guard shielded wire pair should be positioned inside the main cable bundle whose
shield is connected to safety earth ground. The guard shield should be insulated with an
outer jacket. A second Zippertubing
shield can be used as the guard shield.
WARNING The guard terminals are connected to the floating ground of the
7751/7752/7753 module. Do not connect V-source Hi to earth ground. If
V-source Hi is connected to earth ground, guard could float up to 500V.
WARNING Do not connect V-source Hi to earth ground. The 7751/7752/7753 mod-
ule uses a floating ground. This floating ground is not connected to
safety earth ground. Not only will it create a shock hazard by floating
up to 500V, it will also effectively connect a 1nF capacitor across the
V-source, providing energy storage that could ignite an airbag under
fault conditions.
Ground wrapped
cable bundle
Test
Station
Safety Earth
Ground
Keithley
Switching
Module
Keithley 7751, 7752,
7753, or 7702
GUARD
SOURCE SENSE SRC
HI
SRC
HI
SRC
LO
SRC
LO
SEN
HI
SEN
HI
SEN
LO
SEN
LO
A
A
BANK 1
BANK 2
B
B
HI HI
LO LO
HI
SRC
HI
SRC
LO
SRC
LO
SRC
HI
SEN
HI
SEN
LO
SEN
LO
SEN
A
A
LO
I / V
GUARDISOURCEVSOURCEINTERLOCK
OUT
HIGNDGND HI
IN
LOLO
BANK 4
BANK 3
I / V
B
B
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