Avtech AVRQ-5-B, Инструкция по эксплуатации

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CAPACITIVE INTERFERENCE ISSUES
The AVRQ-5-B requires the user to observe a logic-level output signal (typically 5V in
amplitude) in the presence of a 1500 V pulse. This creates ample potential for
interference, which can make it difficult to separate real glitches from measurement
artifacts. This section aims to explain the issues.
When there is no capacitive loading on the output, the AVRQ-4-B can typically exhibit
switching times as fast as 4 ns (10%-90%) for 1500V pulse. If the optocoupler DUT is
an open-collector type with a 1 kilohm pull-up resistance, then a capacitively-coupled
current of 5 mA will be sufficient to cause a 5V spike on the optocoupler output (since
5mA × 1kΩ = 5V). The parasitic capacitance necessary for this to occur may be
calculated from:
I = C dV/dt
C = I dt / dV
C = 5mA × 4 ns / (1500V × (90% - 10%))
C = 0.0167 pF
This is an extremely small capacitance, that can easily occur through parasitic effects
on the daughterboard and external cabling.
Similar effects can occur in the oscilloscope and its probes, through inter-channel
interference.
The worst-case scenario occurs when there is no DUT installed on the daughterboard,
so the logic output is simply floating. The impedance level is very high, so any
capacitively coupled current will generate a substantial voltage. The images below
demonstrate this:
Top: +1.5 kV HV pulse
Bottom: Logic out into open circuit
no DUT, 400 ns/div Top: -1.5 kV HV pulse
Bottom: Logic out into open circuit
no DUT, 400 ns/div
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