AVTECH ELECTROSYSTEMS AVRQ-4-B, Instructions Manual

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INDUCTIVE SPIKES AND PROBE SATURATION
Some care must be taken if using a differential probe to sense the logic output. While a
differential probe can be useful in reducing the possibility of interference from the high-
voltage pulse, it may have strict voltage limits. For example, the Tektronix P6246 is only
suitable for values of VCC2 up to +7V. A non-differential probe may be more suitable if
VCC2 > 7V.
A less obvious problem arises if parasitic inductances or capacitances in the test circuit
cause differential voltage spikes exceeding ±7V. The P6246 can saturate heavily under
those conditions, which can generate apparent glitch-like transients that are not due to
the DUT. Some experimentation may be required by the user in order to identify the
best probing arrangement.
If you are experiencing “unexpected” transients, keep in mind this possibility of probe
saturation.
DC CORRECTNESS ISSUES
The sample daughterboards provided with the AVRQ-4-B are designed to bias the input
side of the optocoupler with a fixed bias during the test. To change it, components must
be physically changed. This can be problematic with certain devices that do not
necessarily power-up to a well-defined state. The NVE IL710 is an example of such a
device – as the IL710 datasheet says:
“To minimize power dissipation, input signals are differentiated and then latched on the
output side of the isolation barrier to reconstruct the signal. This could result in an
ambiguous output state depending on power up, shutdown and power loss sequencing.
Therefore, the designer should consider including an initialization signal in the start-up
circuit. Initialization consists of toggling the input either high then low, or low then high.”
The user may wish to add power-up toggle circuitry, or an oscillator, in order to
correctly initialize and drive such devices.
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