F-12 Measurement Considerations
For voltmeters that do not use guarding, the triax input cable is a primary source of capaci-
tance. Inside the cable, the insulation between the HI and LO terminals form a capacitor that
shunts the input of the voltmeter. The longer the cable, the larger the capacitance.
The guarded triax connectors of the Remote PreAmp minimize the problem of capacitance
in 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. With HI and guard at the same volt-
age, there is no charge-discharge capacitive action to slow down the measurement. However, in
reality, there is a small voltage differential between HI and guard (<1mV). Therefore, there will
be a little capacitive action due to the cables. The best way to minimize this capacitance is to
use short triax cables.
R
E
C
V
M
Measured
Source
τ = RC
6430
Voltmeter
LO
HI
Figure F-6
Effects of input
capacitance
RC
0 1.0 2.0 3.0 4.0 5.0
Time
0
63
100
Percent
of Final
Value
Figure F-7
Settling time
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