Tektronix IsoVu TIVM Series, User Manual

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Reference information
IsoVu measurement system block diagram
The following fi gure shows a block diagram of the IsoVu measurement system.
Figure 25: Block diagram
The common mode resistance and capacitance to earth ground is shown in the
fi gure. (See Figure 25.) The common mode resistance is shown as R parasitic and
is essentially in fi nite with the IsoVu measurement system since it is galvanically
isolated and can be ignored. The common mode coupling capacitance to earth
ground and the surrounding circuit is shown as the Parasitic Bridging Capacitance
(
C parasitic ). This parasitic capacitance will be approximately 2 pF when the
sensor head is placed six (6) inches (15.25 cm) above a ground plane.
To minimize the effects of common mode capacitive loading consider the
following items:
Whenever possible, choose a reference point in the circuit-under-test that is
static potential with respect to earth ground.
Connect the coaxial (common) shield of the sensor tip cable to the lowest
impedance point of the circuit.
Increasing the physical distance between the sensor head and any conductive
surface will reduce the parasitic capacitance.
When using multiple IsoVu systems to measure different points in the circuit
that do not have the same common mode voltages, keep the sensor heads
separated to minimize the capacitive coupling.
36 TIVM Series Measurement System User Manual