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Volts and Ohms Measurements
3-13
Guarding the circuit minimizes these effects by driving the inner shield of the triax cable at
signal potential, as shown in Figure 3-8. Here, a unity gain amplifier with a high input imped-
ance and low output impedance is used. Since the center conductor (HI) and the inner shield
(Guard) of the cable are at virtually the same potential, the potential across R
L
is zero, so no cur-
rent flows. Also, with a zero potential across C
L
, there is no capacitor charging process to slow
down the measurement response.
For the volts and ohms functions, the input of Model 6514 places the driven guard on the
inner shield of the triax cable when GRD is enabled.
Inner Shield
Triax Cable
R
S
HI
6514 Input
LO
E
S
R
L
C
L
Source
Center
Conductor
A = I
Guard
150k
Ω
Figure 3-8
Guarded input cable
Summary of Contents for 6514
Page 203: ...14 CommonCommands...
Page 207: ...15 SCPISignalOriented MeasurementCommands...
Page 235: ...18 PerformanceVerification...
Page 254: ...19 Calibration...
Page 274: ...20 RoutineMaintenance...
Page 278: ...A Specifications...
Page 282: ...B StatusandErrorMessages...
Page 288: ...C GeneralMeasurement Considerations...
Page 294: ...D DDCEmulationCommands...
Page 299: ...E ExamplePrograms...
Page 306: ...F IEEE 488BusOverview...
Page 320: ...G IEEE 488andSCPI ConformanceInformation...
Page 324: ...H CalibrationOptions...