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Chapter 2
Using the Module
•
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 NI PXIe-4302/4303 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.
1
Optional Buffered Mode IIR Filtering
In addition to the sharp anti-alias filters used in buffered mode, the NI PXIe-4302/4303 also
provides optional fourth order elliptical lowpass IIR filters that can be used in Buffered Mode.
These lowpass filters have fixed cut-off frequencies that are not related to the sample rate and
are intended to replicate placing an analog filter on the input. For example, if you want to filter
out 60 Hz power line noise, you can select the digital 2 Hz lowpass filter and still use any desired
sample rate.
The optional lowpass filter is in addition to the anti-alias filter. Therefore, the optional lowpass
filter is only meaningful in the passband of the anti-alias filter. The anti-alias filter will still reject
frequencies beyond the Nyquist frequency.
By default, the optional lowpass digital filter is not enabled. Each channel in a Buffered Mode
task can independently be configured to use any of the available filters that the
NI PXIe-4302/4303 supports. Refer to
Digital AI Filtering
in the
NI-DAQmx Help
for more
information about how to configure the filter used. Refer to the
NI PXIe-4302/4303
Specifications
for more information about the cut-off frequencies supported and response
characteristics of these filters.
Filter Settling Time
When a lowpass filter is first enabled, it takes time for the filter to settle to a correct value.
Software automatically waits a fixed amount of time after enabling the filter and before starting
a task to prevent showing invalid data. The amount of time software waits is dependent on the
cut-off frequency of the filter selected. The lower the cut-off frequency, the longer software must
wait for the filter to settle. Refer to the
NI PXIe-4302/4303 Specifications
for more information.
Filter Group Delay
Like all filters, the optional lowpass filter has group delay associated with it. Since the optional
filters are IIR, the group delay is a function of both cut-off frequency and the frequency of the
signal being attenuated. Since the group delay is not constant for all input frequencies, it cannot
be automatically compensated for and will therefore appear as an additional delay on the input.
Refer to the
NI PXIe-4302/4303 Specifications
for more information.
1
The maximum sample rate of the NI PXIe-4302/4303 is 51.2 kS/s. In addition to the fixed analog input
delay, you must also account for the digital-filter group delay. For 51.2 kS/s, the digital filter group delay
is .
If a 10 V range is used, total delay is 1.79837 ms + 6.77
μ
s = 1.80514 ms. Refer to the
NI PXIe-4302/4303
Specifications
for more information.
1.79742 ms
48.5 S
51.2 kS/s
----------------------
+
1.79837 ms
=
