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3-3
Passband
The signals within the passband have frequency-dependent gain or attenuation. The small
amount of variation in gain with respect to frequency is called the passband flatness. The digital
anti-alias filters of the PXIe-4480/4481 adjust the frequency range of the passband to match the
sample rate. Therefore, the amount of gain or attenuation at a given frequency depends on the
sample rate.
Stopband
The digital anti-alias filter significantly attenuates all signals above the stopband frequency to
prevent aliasing. Therefore, the stopband frequency scales precisely with the sample rate. The
stopband rejection is the minimum amount of attenuation applied by the anti-alias filter to all
signals with frequencies within the stopband.
Alias-Free Bandwidth
Any signal that appears in the alias-free bandwidth of the PXIe-4480/4481 is not an aliased
artifact of signals at a higher frequency. The alias-free bandwidth is defined by the ability of the
filter to reject frequencies above the stopband frequency, and it is equal to the sample rate minus
the stopband frequency.
Filter Group Delay
The anti-alias digital filtering performed by the PXIe-4480/4481 produces a delay of several
samples worth of time between when an event occurs on the input signal going into the
PXIe-4480/4481 and when the data associated with that event is available at the output of the
acquisition and filtering process. This delay is called the group delay. In order to simplify the
process of acquiring data from the PXIe-4480/4481 modules and correlating that data with data
from other modules, the PXIe-4480/4481 compensates for this group delay in the following
ways:
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The Sample Clock output from the PXIe-4480/4481 is generated at the point in time when
the input signal is valid at the ADC input pins. When acquiring data, the PXIe-4480/4481
generates a Sample Clock, then waits for the data associated with that Sample Clock to be
acquired, then returns that data. As a result, any other acquisitions timed with this Sample
Clock line up with the data returned by the PXIe-4480/4481.
•
Any triggers generated or received by the PXIe-4480/4481 are interpreted based on their
relationship to the Sample Clock being generated. For example, a Start Trigger that starts
an acquisition results in data from the next Sample Clock being returned as the first point
in the acquisition. Refer to the
section for more details about
how this affects analog trigger events.
•
On-demand software sampling returns a single sample from an acquisition running at
the maximum supported sample rate of the module. For any on-demand, software-timed
acquisition the PXIe-4480/4481 waits for the group delay to elapse before returning the
sample. As a result, the data returned aligns closely in time with when the data was
requested and is delayed by the sum of the analog input delay and digital filter delay.