USB-2533 User's Guide
Functional Details
30
Noise in measurements only decreases as the square root of the number of measurements—reducing RMS
noise significantly may require many samples. Thus, averaging is suited to low-speed applications that can
provide many samples.
Only random noise is reduced or eliminated by averaging. Averaging does not reduce or eliminate periodic
signals.
Digital I/O
Twenty-four TTL-level digital I/O lines are included in each USB-2533. You can program digital I/O in 8-bit
groups as either inputs or outputs and scan them in several modes (see "
below). You can
access input ports asynchronously from the PC at any time, including when a scanned acquisition is occurring.
Digital input scanning
Digital input ports can be read asynchronously before, during, or after an analog input scan.
Digital input ports can be part of the scan group and
scanned along with analog input channels
. Two
synchronous modes are supported when digital inputs are scanned along with analog inputs.
Refer to "
Example 4: Sampling digital inputs for every analog sample in a scan group
" on page 28 for more
information.
In both modes, adding digital input scans has no affect on the analog scan rate limitations.
If no analog inputs are being scanned, the digital inputs can be scanned at up to 12 MHz.
Triggering
Triggering can be the most critical aspect of a data acquisition application. The USB-2533 supports the
following trigger modes to accommodate certain measurement situations.
Hardware analog triggering
The USB-2533 uses true analog triggering in which the trigger level you program sets an analog DAC, which is
then compared in hardware to the analog input level on the selected channel. This guarantees an analog trigger
latency that is less than 1 µs.
You can select any analog channel as the trigger channel, but the selected channel must be the first channel in
the scan. You can program the trigger level, the rising or falling edge, and hysteresis.
A note on the hardware analog level trigger and comparator change state
When analog input voltage starts near the trigger level, and you are performing a rising or falling hardware
analog level trigger, the analog level comparator may have already tripped before the sweep was enabled. If this
is the case, the circuit waits for the comparator to change state. However, since the comparator has already
changed state, the circuit does not see the transition.
To resolve this problem, do the following:
1.
Set the analog level trigger to the threshold you want.
2.
Apply an analog input signal that is
more than
2.5% of the full-scale range
away from the desired
threshold
. This ensures that the comparator is in the proper state at the beginning of the acquisition.
