National Instruments Data Acquisition Device NI USB-621x - Manual (Page 49)

Chapter 4 Analog Input
NI USB-621x User Manual 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.
You can specify either an internal or external signal as the source of
AI Convert Clock. You also can specify whether the measurement sample
begins on the rising edge or falling edge of AI Convert Clock.
By default, NI-DAQmx chooses the fastest conversion rate possible based
on the speed of the A/D converter and adds 10
μs of padding between each
channel to allow for adequate settling time. This scheme enables the
channels to approximate simultaneous sampling and still allow for
adequate settling time. If the AI Sample Clock rate is too fast to allow
for this 10
μs of padding, NI-DAQmx chooses the conversion rate so that
the AI Convert Clock pulses are evenly spaced throughout the sample.
To explicitly specify the conversion rate, use AI Convert Clock Rate
DAQmx Timing property node or function.
Caution Setting the conversion rate higher than the maximum rate specified for your
device will result in errors.
Using an Internal Source
One of the following internal signals can drive AI Convert Clock:
• AI Convert Clock Timebase (divided down)
• Counter n Internal Output
A programmable internal counter divides down the AI Convert Clock
Timebase to generate AI Convert Clock. Started by AI Sample Clock, the
counter counts down to zero, produces an AI Convert Clock, reloads itself,
and repeats the process until the sample is finished. It then reloads itself in
preparation for the next AI Sample Clock pulse.
Using an External Source
Use any input PFI line as the source of AI Convert Clock.
Routing AI Convert Clock to an Output Terminal
You can route AI Convert Clock (as an active low signal) out to any output
PFI terminal.