Chapter 4
Analog Input
©
National Instruments
4-31
X Series User Manual
AI Convert Clock Signal
Use the AI Convert Clock (ai/ConvertClock) signal to initiate a single A/D
conversion on a single channel. A sample (controlled by the AI Sample
Clock) consists of one or more conversions.
Yo
u
can specify either an internal or external signal as the so
u
rce of
AI Convert Clock. Yo
u
also can specify whether the meas
u
rement sample
begins on the rising edge or falling edge of AI Convert Clock.
With NI-DAQmx, the driver chooses the fastest conversion rate possible
based on the speed of the A/D converter and adds 10
μ
s of padding between
each channel to allow for adeq
u
ate settling time. This scheme enables the
channels to approximate sim
u
ltaneo
u
s sampling and still allow for
adeq
u
ate settling time. If the AI Sample Clock rate is too fast to allow for
this 10
μ
s of padding, NI-DAQmx chooses the conversion rate so that the
AI Convert Clock p
u
lses are evenly spaced thro
u
gho
u
t the sample.
To explicitly specify the conversion rate,
u
se
AI Convert Clock Rate
DAQmx Timing
property node or f
u
nction.
Caution
Setting the conversion rate higher than the maxim
u
m rate specified for yo
u
r
device will res
u
lt in errors.
Using an Internal Source
One of the following internal signals can drive AI Convert Clock:
•
AI Convert Clock Timebase (divided down)
•
Co
u
nter
n
Internal O
u
tp
u
t
•
Change Detection Event
•
Co
u
nter
n
Sample Clock
•
AO Sample Clock (ao/SampleClock)
•
DI Sample Clock (di/SampleClock)
•
DO Sample Clock (do/SampleClock)
A programmable internal co
u
nter divides down the AI Convert Clock
Timebase to generate AI Convert Clock. The co
u
nter is started by AI Sample
Clock and contin
u
es to co
u
nt down to zero, prod
u
ces an AI Convert Clock,
reloads itself, and repeats the process
u
ntil the sample is finished. It then
reloads itself in preparation for the next AI Sample Clock p
u
lse.
Several other internal signals can be ro
u
ted to AI Convert Clock thro
u
gh
internal ro
u
tes. Refer to
Device Routing in MAX
in the
NI-DAQmx Help
or
the
LabVIEW Help
for more information.
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