Continuous Waveform Acquisition Using Scan Clock Gating
PrintDescription
In some situations it may be desirable to acquire data only while an external signal is high or low. Scan clock gating uses an external TTL signal to "gate" the analog input, effectively pausing and resuming an acquisition based on the selected pause condition. If the gate signal is high, then analog input scans are acquired at the rate specified. If the gate is low, no data is acquired. The acquisition can also be paused while the gate signal is high and acquiring data while the signal is low. This feature is useful for implementing pauses in the acquisition or for retriggered operations where reconfiguring the trigger is too slow.Using scan clock gating, it is possible to make an analog scan clock gate such that data is only acquired while the input signal meets certain voltage requirements. Because any TTL signal can be used for scan clock gating, the ATCOUT signal, which indicates whether the analog trigger condition is met (assuming that the board supports analog triggering) can be used to only take data when the input signal is above, below, or between specified levels, as shown in Figure 1.
Figure 1: Using ATCOUT as a Scan Clock Gate
Digital Scan Clock Gating
LabVIEW:
Call AI Trigger Config with trigger type = "digital scan clock gating" (4) and trigger or pause condition = "pause scanning while below level or TTL low" (8) or "pause scanning while above level or TTL high" (7).
NI-DAQ:
Call Select_Signal with signal = ND_IN_INTERNAL_GATE and source = ND_PAUSE_ON_LOW or ND_PAUSE_ON_HIGH.
Analog Scan Clock Gating
LabVIEW:
Call AI Trigger Config with trigger type = "analog scan clock gating" (5) and trigger or pause condition = "pause scanning while inside window" (6), "pause scanning while outside window" (7), "pause scanning while above level or TTL high" (7), or "pause scanning while below level or TTL low" (8).
NI-DAQ:
Call Configure_HW_Analog_Trigger, then Select_Signal with signal = ND_IN_EXTERNAL_GATE and source = ND_PAUSE_WHILE_LOW or ND_PAUSE_ON_HIGH.
Common Applications
Acquiring data only within "interesting" ranges.
Acquiring data repeatedly only when certain external events occur.
Related Links:
Continuous Analog Acquisition Using Digital Scan Clock Gating
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