National Instruments Data Acquisition Device NI USB-621x, User Manual

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Chapter 4 Analog Input
© National Instruments Corporation 4-17 NI USB-621x User Manual
Using a Delay from Sample Clock to Convert Clock
When u sing an internally generated AI Convert Clock, yo u also can specify
a config u rable delay from AI Sample Clock to the first AI Convert Clock
p u lse within the sample. By defa u lt, this delay is three ticks of AI Convert
Clock Timebase.
Fig u re 4-9 shows the relationship of AI Sample Clock to AI Convert Clock.
Figure 4-9. AI Sample Clock and AI Convert Clock
Other Timing Requirements
The sample and conversion level timing of USB-621 x devices work s u ch
that clock signals are gated off u nless the proper timing req u irements are
met. For example, the device ignores both AI Sample Clock and AI
Convert Clock
u ntil it receives a valid AI Start Trigger signal. Once the
device recognizes an AI Sample Clock p u lse, it ignores s u bseq u ent AI
Sample Clock p u lses u ntil it receives the correct n u mber of AI Convert
Clock p u lses.
Similarly, the device ignores all AI Convert Clock p u lses u ntil it recognizes
an AI Sample Clock p u lse. Once the device receives the correct n u mber of
AI Convert Clock p u lses, it ignores s u bseq u ent AI Convert Clock p u lses
u ntil it receives another AI Sample Clock. Fig u res 4-10, 4-11, 4-12,
and 4-13 show timing seq u ences for a fo u r-channel acq u isition ( u sing AI
channels 0, 1, 2, and 3) and demonstrate proper and improper seq u encing
of AI Sample Clock and AI Convert Clock.
AI Convert Clock Time bas e
AI Sa mple Clock
AI Convert Clock
Del a y
From
Sa mple
Clock
Convert
Period