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F-4
Measurement Considerations
An example of guarding as applied to an ionization chamber is shown in Figure F-2. An
unguarded ionization chamber and the corresponding equivalent circuit are shown in
Figure F-2A. The equivalent circuit shows that the full bias voltage appears across the insulator
leakage resistance (R
L
) and thus, a leakage current (I
L
) will be added to the measured ion cur-
rent (I
M
= I
C
+ I
L
). The leakage resistance is primarily due to the insulator of the ionization
chamber.
In Figure F-2B, a metal guard ring is added to the ionization chamber. This guard ring is
connected to the driven guard of the Remote PreAmp. This circuit splits the leakage resistance
into two parts; R
L1
and R
L2
. The driven guard is at almost the same voltage potential as output
HI. The voltage difference is <1mV, and is known as the voltage burden of the Remote
PreAmp. Since the top and bottom of R
L1
are at nearly the same potential, no signi
fi
cant cur-
rent will
fl
ow through it.
In a similar manner, guarding may also be necessary to prevent leakage current in test
fi
x-
tures. See
Cable guard
and Figure 5-11 in Section 5 for details.
In/Out
HI
In/Out
LO
Remote
PreAmp
I
M
V
S
R
L
I
L
I
C
In/Out
HI
Remote
PreAmp
Guard
In/Out
LO
I
M
V
S
I
M
V
S
x1
I
M
V
S
x1
R
L1
R
L2
<100
µ
V
A. Unguarded ionization chamber
B. Guarded ionization chamber
Equivalent Circuit
Equivalent Circuit
Figure F-2
Guarding an
ionization
chamber
Summary of Contents for 6430
Page 26: ......
Page 32: ......
Page 78: ...2 14 Connections ...
Page 98: ...3 20 Basic Source Measure Operation ...
Page 138: ...5 30 Source Measure Concepts ...
Page 156: ...6 18 Range Digits Speed and Filters ...
Page 168: ...7 12 Relative and Math ...
Page 176: ...8 8 Data Store ...
Page 202: ...9 26 Sweep Operation ...
Page 248: ...11 22 Limit Testing ...
Page 310: ...16 6 SCPI Signal Oriented Measurement Commands ...
Page 418: ...17 108 SCPI Command Reference ...
Page 450: ...18 32 Performance Verification ...
Page 477: ...A Specifications ...
Page 489: ...B StatusandErrorMessages ...
Page 498: ...B 10 Status and Error Messages ...
Page 499: ...C DataFlow ...
Page 503: ...D IEEE 488BusOverview ...
Page 518: ...D 16 IEEE 488 Bus Overview ...
Page 519: ...E IEEE 488andSCPI ConformanceInformation ...
Page 523: ...F MeasurementConsiderations ...
Page 539: ...G GPIB488 1Protocol ...
Page 557: ......