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
Содержание 6430
Страница 26: ......
Страница 32: ......
Страница 78: ...2 14 Connections ...
Страница 98: ...3 20 Basic Source Measure Operation ...
Страница 138: ...5 30 Source Measure Concepts ...
Страница 156: ...6 18 Range Digits Speed and Filters ...
Страница 168: ...7 12 Relative and Math ...
Страница 176: ...8 8 Data Store ...
Страница 202: ...9 26 Sweep Operation ...
Страница 248: ...11 22 Limit Testing ...
Страница 310: ...16 6 SCPI Signal Oriented Measurement Commands ...
Страница 418: ...17 108 SCPI Command Reference ...
Страница 450: ...18 32 Performance Verification ...
Страница 477: ...A Specifications ...
Страница 489: ...B StatusandErrorMessages ...
Страница 498: ...B 10 Status and Error Messages ...
Страница 499: ...C DataFlow ...
Страница 503: ...D IEEE 488BusOverview ...
Страница 518: ...D 16 IEEE 488 Bus Overview ...
Страница 519: ...E IEEE 488andSCPI ConformanceInformation ...
Страница 523: ...F MeasurementConsiderations ...
Страница 539: ...G GPIB488 1Protocol ...
Страница 557: ......