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 flow 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 2GΩ, then the leakage current will
be <0.5pA (<1mV / 2GΩ = 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 / 2GΩ = 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 five time constants to
achieve an accurate reading. For example, if R = 100GΩ and C = 10pF, RC time constant
would be 1 second. If 1% accuracy is required, a single measurement would require at least five
seconds.
Source
R
s
V
E
s
R
IN
SourceMeter
% Error =
100R
S
R
S
+ R
IN
Figure F-5
Meter loading
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