Chapter 4
Analog Input
4-14
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AI Sample Clock Signal
Use the AI Sample Clock (ai/SampleClock) signal to initiate a set of
meas
u
rements. Yo
u
r USB-621
x
device samples the AI signals of every
channel in the task once for every AI Sample Clock. A meas
u
rement
acq
u
isition consists of one or more samples.
Yo
u
can specify an internal or external so
u
rce for AI Sample Clock. Yo
u
also can specify whether the meas
u
rement sample begins on the rising edge
or falling edge of AI Sample Clock.
Using an Internal Source
One of the following internal signals can drive AI Sample Clock:
•
Co
u
nter
n
Internal O
u
tp
u
t
•
AI Sample Clock Timebase (divided down)
•
A p
u
lse initiated by host software
A programmable internal co
u
nter divides down the sample clock timebase.
Using an External Source
Use any inp
u
t PFI line as the so
u
rce of AI Sample Clock.
Routing AI Sample Clock to an Output Terminal
Yo
u
can ro
u
te AI Sample Clock o
u
t to any o
u
tp
u
t PFI terminal. This p
u
lse
is always active high.
Yo
u
can specify the o
u
tp
u
t to have one of two behaviors. With the p
u
lse
behavior, yo
u
r DAQ device briefly p
u
lses the PFI terminal once for every
occ
u
rrence of AI Sample Clock.
With level behavior, yo
u
r DAQ device drives the PFI terminal high d
u
ring
the entire sample.
Other Timing Requirements
Yo
u
r DAQ device only acq
u
ires data d
u
ring an acq
u
isition. The device
ignores AI Sample Clock when a meas
u
rement acq
u
isition is not in
progress. D
u
ring a meas
u
rement acq
u
isition, yo
u
can ca
u
se yo
u
r DAQ
device to ignore AI Sample Clock
u
sing the AI Pa
u
se Trigger signal.
A co
u
nter on yo
u
r device internally generates AI Sample Clock
u
nless yo
u
select some external so
u
rce. AI Start Trigger starts this co
u
nter and either
