Chapter 3
Hardware Overview
PCI-4451/4452/4453/4454 User Manual
3-8
www.ni.com
Note
The PCI-4451 boots in a mode with the outputs disabled and infinitely (
∞
)
attenuated. Although these functions appear similar, they are distinct and are implemented
to protect your external equipment from startup transients.
♦
PCI-4453
This device does not feature any attenuation setting capability and boots in
a mode with outputs disabled.
Trigger
In addition to supporting internal software triggering and external digital
triggering to initiate a data acquisition sequence, the PCI-445X 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 resolution for a given input range.
Refer to Table 3-1 for more information.
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,
you can configure the analog input section to acquire a given number of
samples after the analog input signal crosses a specific threshold.
Due to the nature of delta-sigma converters, the triggering circuits operate
on the digital output of the converter. Since the trigger is generated at the
output of the converter, triggers can occur only when a sample is actually
generated. Placing the triggering circuits on the digital side of the converter
does not affect most measurements unless an analog output is generated
based on the input trigger. In this case, you account for the inherent delays
of the finite impulse response (FIR) filters internal to the delta-sigma
converters. The delay through the input converter is 42 sample periods,
while the delay through the output converter is 34.6 ±0.5 sample periods.
Note that the input and output sample periods may differ.
During repetitive sampling of a waveform, you might observe jitter due
to the uncertainty of where a trigger level falls compared to the actual
digitized data. Although this trigger jitter is never greater than one sample
period, it can seem quite bad when the sample rate is only twice the