Chapter 4
Analog Input
4-16
ni.com
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.
By defa
u
lt, NI-DAQmx 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
A programmable internal co
u
nter divides down the AI Convert Clock
Timebase to generate AI Convert Clock. Started by AI Sample Clock, the
co
u
nter co
u
nts 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.
Using an External Source
Use any inp
u
t PFI line as the so
u
rce of AI Convert Clock.
Routing AI Convert Clock to an Output Terminal
Yo
u
can ro
u
te AI Convert Clock (as an active low signal) o
u
t to any o
u
tp
u
t
PFI terminal.
