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Chapter 3 Hardware Overview
PCI-4451/4452 User Manual 3-6
©
National Instruments Corporation
Note The device boots in a mode with the outputs disabled
AND
infinitely (∞) attenuated.
Although these functions appear similar, they are quite distinct and are
implemented to protect your external equipment from startup transients.
When the DACs no longer have data written to them, they automatically
retransmit the last data point they received. If you are expecting the data to return
to 0 V or any other voltage level, you
MUST
append the data to make it do so.
All data written to the DACs are interpreted as two’s complement format.
In two’s complement mode, data values written to the analog output
channel are either positive or negative.
Trigger
In addition to supporting internal software triggering and external digital
triggering to initiate a data acquisition sequence, the PCI-4451/4452 also
supports analog level triggering. You can configure the trigger circuit to
monitor any one of the analog input channels to generate the level trigger.
Choosing an input channel as the level trigger channel does not influence
the input channel capabilities. The level trigger circuit compares the full
16 bits of the programmed trigger level with the digitized 16-bit sample.
The trigger-level range is identical to the analog input voltage range. The
trigger-level resolution is the same as the precision for a given input range.
Refer to Table 3-1.
The trigger circuit generates an internal digital trigger based on the input
signal and the user-defined trigger levels. Any of the timing sections of the
DAQ-STC can use this level trigger, including the analog input, analog
output, RTSI, and general-purpose counter/timer sections. For example,
Table 3-2.
Actual Range and Measurement Precision
Attenuation
Linear
Attenuation
dB
Range Precision
1
1.0 0 dB ±10.0 V 305.18 µV
10 20 dB ±1.00 V 30.158 µV
100 40 dB ±0.100 V 3.0518 µV
∞ ∞ dB 0 V 0 V
1
The value of 1 LSB of the 16-bit DAC; that is, the voltage increment corresponding to a
change of one count in the DAC 16-bit count.
See Appendix A,
Specifications
, for absolute maximum ratings.
User.book Page 6 Tuesday, April 14, 1998 10:20 AM