Tektronix IsoVu TIVM Series, User Manual

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Application examples
In this example, the gate drive voltage could be about 5 V but usually has some
ringing and overshoot which are important to characterize. For this measurement,
it is appropriate to use a 10X tip with a 10 V
p-p
input (on the 1X range) so that
the signal is fully resolved and is within the dynamic range of the measurement
system.
To get the best CMRR from your TIVM Series IsoVu measurement system, pay
careful attention when connecting the measurement system to the DUT. This
connection should preserve the signal
fi delity and shield the signal from unwanted
interference. To achieve the best performance from the measurement system, use
an MMCX connector as close to the test point as possible. The MMCX connectors
are available from a number of vendors and are relatively inexpensive. The key
attributes of these connectors, which make them excel for this application, are
their compact footprint and solid metal body. The solid metal body and gold
contacts provide a well-shielded signal path.
The IsoVu input provides a fl oating differential measurement with differential
input resistance from 50 Ω to 2.5 k Ω , depending on the tip attenuation. In the
VGS measurement example the 10X tip with a 500 Ω input impedance is used.
The common mode resistance is extremely high, greater than G Ω , in parallel
with a small capacitance from the tip cable shield to ground, typically 2 pF or
less. Keep these impedances in mind when determining how to connect the
measurement system to the DUT. The source node in the half bridge circuit is a
very low impedance point, and is the point that should be used to drive the tip
cable shield capacitance. The gate driver output, which is also a low impedance
node (but not as low impedance as the source), should be used to drive the center
contact of the sensor tip cable. The input impedance of the sensor tip cable for the
10X tip is 500
Ω relative to the shield – not to ground.
An example of a measurement that is possible with IsoVu is measuring the
high-side turn-on characteristics shown in the following
fi gure.
Figure 16: High-side turn-on characteristics
24 TIVM Series Measurement System User Manual