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Measurement Considerations
F-11
Cable leakage resistance
For a voltage measurement, leakage resistance in a triax cable (from HI to LO) shunts the
voltage source to be measured. If the voltage source has a very high-impedance, the resultant
leakage current could be high enough to corrupt the measurement.
The Remote PreAmp uses guarded triax connectors to, for the most part, eliminate the prob-
lem of leakage current in triax cables. The center conductor (HI) is surrounded by the inner
shield, which is cable guard. Ideally, guard is at the same potential as the HI terminal, and
therefore, no leakage current will
fl
ow through the insulation. However, in reality, there is a
small voltage differential between HI and guard (<1mV). Therefore, there will be a small leak-
age current through the cable. If the insulation resistance is 2G
Ω
, then the leakage current will
be <0.5pA (<1mV / 2G
Ω
= 0.5pA).
For voltmeters that do not use guarding, the leakage current would be dependent on the volt-
age seen at input HI. For example, with input HI at 10V, the leakage current would be 5nA
(10V / 2G
Ω
= 5nA). This leakage is 10,000 times higher than the Model 6430 Remote
PreAmp.
Input capacitance (settling time)
The settling time of the circuit is particularly important when making volts measurements of
a source that has high internal resistance (Figure F-6).
The shunt capacitance (C) has to fully charge before an accurate voltage measurement can
be made by VM of Model 6430. The time period for charging the capacitor is determined by
the RC time constant (one time constant,
τ
= RC), and the familiar exponential curve of
Figure F-7 results. Therefore, it becomes necessary to wait four or
fi
ve time constants to
achieve an accurate reading. For example, if R = 100G
Ω
and C = 10pF, RC time constant
would be 1 second. If 1% accuracy is required, a single measurement would require at least
fi
ve
seconds.
Source
Rs
V
Es
R
IN
SourceMeter
% Error =
100R
S
R
S
+ R
IN
Figure F-5
Meter loading
Содержание 6430
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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 ...
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