Programming instructions

Chapter 20 DSA Devices
DAQ Hardware Overview Guide 20-2 www.ni.com
PCI-445 X and NI 455 X Analog Input Channels
The 445X and 455X devices contain simultaneously sampled differential
analog input channels based on delta-sigma ADCs. These converters use an
oversampling and filtering technique to antialias the input signal. The
converters produce 16-bit resolution readings. Each channel has software
configurable coupling settings. The PCI-4451, PCI-4452, NI 4551, and
NI 4552 also has software configurable gain settings. All DSA devices
except for the PDI-4453 and PCI-4454 have software configurable gain.
These gains range from −20 to 60 dB in steps of 10 dB.
The signal range of each input channel is ±10 V when set to DC coupling
and 0 dB gain. When the input channels are set to AC coupling and 0 dB
gain, the signal range is ±10 VAC with a maximum input voltage of
42 VDC.
Data Acquisition
For data acquisition operations, the devices scan the channels in ascending
order, but you can individually include or exclude a channel in a scan.
For example, you can configure a data acquisition operation that scans
channels 1 and 3 or channels 0, 1, and 2.
DSA devices format samples in a left-justified format in 32-bit data words.
This means that the most significant bits of the data word contain the bits
generated by the converter. When allocating data buffers, be sure to account
for the 32-bit data width. Even though a number of DAQ and SCAN
functions are declared to accept pointers to 16-bit data buffers, you should
pass the pointers to 32-bit data buffers for the DSA devices.
You can configure a data acquisition operations to use any of the following
additional modes:
• Posttrigger mode
• Pretrigger mode
• Double-buffered mode
Posttrigger mode collects a specified number of samples after the device
receives a trigger. Refer to the start trigger discussion in the data acquisition
timing section later in this chapter for details. After the user-specified
buffer is full, the data acquisition stops.