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Appendix B
An Important Note Regarding Hardware Analog Level Trigger and
Comparator Change State
Issue:
When the starting out analog input voltage is near the trigger level, and you are performing a rising
[or falling] hardware analog level trigger, it is possible that the analog level comparator will have
already tripped, i.e., to have tripped before the sweep was enabled. If this is the case, the circuit will
wait for the comparator to change state. However, since the comparator has already changed state,
the circuit will not see the transition.
Solution:
(1) Set the analog level trigger to the desired threshold.
(2) Apply an analog input signal that is more than 2.5% of the full-scale range away from the
desired threshold. This ensures that the comparator is in the proper state at the beginning
of the acquisition.
(3) Bring the analog input signal toward the desired threshold. When the input signal is at the
threshold (± some tolerance) the sweep will be triggered.
(4) Before re-arming the trigger, again move the analog input signal to a level that is more
than 2.5% of the full-scale range away from the desired threshold.
Example:
o an engineer is using the ±2V full-scale range (gain = 5)
o he desires to trigger at +1V on the rising edge
o he sets the analog input voltage to an initial start-value which is
Appendix B
less than +0.9V (1V – (2V * 2 * 2.5%)).
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Hardware Analog Level Trigger
Hardware Analog Level Trigger
B-1
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