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3-1
3
Signal Acquisition
This chapter contains information about signal acquisition concepts, including analog-to-digital
converters, operation modes, filtering, timing, triggering, and synchronization.
Analog-to-Digital Converter
The PXIe-4480/4481 analog-to-digital converters (ADCs) use the Delta-Sigma modulation
conversion method. This approach involves oversampling the input signal and then decimating
and filtering the resulting data to achieve the desired sample rate.
Nyquist Frequency and Nyquist Bandwidth
Any sampling system, such as an ADC, is limited in the bandwidth of the signals it can measure.
Specifically, a sampling rate of
fs
can represent only signals with frequencies lower than
fs
/2.
This maximum frequency is known as the
Nyquist frequency
. The bandwidth from 0 Hz to the
Nyquist frequency is the
Nyquist bandwidth
.
Operation Modes
The PXIe-4480/4481 supports two acquisition modes: frequency domain mode and time domain
mode.
In a frequency domain mode acquisition, oversampled data is decimated to the requested user
sample rate, and digital filters are applied to filter out frequency content above the Nyquist
frequency. Frequency domain mode is used any time the requested sample rate is between
100 S/s and 1.25 MS/s. Refer to the
Frequency Domain Mode Acquisitions
section for details on
the acquisition process and filters used in frequency domain mode.
The PXIe-4480/4481 also supports time domain mode, in which the oversampled data is loosely
filtered so that a maximum amount of bandwidth can be measured. Data may or may not be
decimated. Time domain mode is used any time the requested sample rate is above 1.25 MS/s
and can operate at up to 20 MS/s. Refer to the
and the
sections for details on the acquisition process and filters used in
time domain mode.
The ability for the module to sample at a requested rate is limited by the module’s sample rate
generation circuitry and, as a result, a sample rate resolution specification is provided. The
sample rate resolution specification indicates how many samples per second are between
two adjacent sample rates that the module is capable of producing. Because of the decimation