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3-12
Volts and Ohms Measurements
Guarding input cable
Guarding should be used for high-impedance measurements and for low voltage measure-
ments that use long input cables.
To better understand the concept of guarding, review the unguarded circuit shown in Figure
3-7. E
S
and R
S
represents the resistance and voltage components of the source, and R
L
and C
L
represents the leakage resistance and cable capacitance of the triax input cable. The equivalent
circuit shows the divider that is formed. If R
S
is large enough, the divider will significantly atten-
uate the voltage seen at the input of Model 6514 (see “Cable Leakage Resistance”). Also, R
S
and
the cable capacitance (C
L
) could create a long RC time constant resulting in a slow measurement
response (see “Input Capacitance”).
RC
0
1.0
2.0
3.0
4.0
5.0
Time
0
63
100
Percent
of Final
Value
Figure 3-6
Settling time
Inner Shield
Triax Cable
R
S
HI
To 6514
Input
LO
E
S
R
L
C
L
Source
Center
Conductor
Equivalent Circuit
R
S
HI
To 6514
Input
LO
E
S
R
L
C
L
Figure 3-7
Unguarded input cable
Содержание 6514
Страница 203: ...14 CommonCommands...
Страница 207: ...15 SCPISignalOriented MeasurementCommands...
Страница 235: ...18 PerformanceVerification...
Страница 254: ...19 Calibration...
Страница 274: ...20 RoutineMaintenance...
Страница 278: ...A Specifications...
Страница 282: ...B StatusandErrorMessages...
Страница 288: ...C GeneralMeasurement Considerations...
Страница 294: ...D DDCEmulationCommands...
Страница 299: ...E ExamplePrograms...
Страница 306: ...F IEEE 488BusOverview...
Страница 320: ...G IEEE 488andSCPI ConformanceInformation...
Страница 324: ...H CalibrationOptions...