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Chapter 3
Signal Acquisition
repeats until all requested samples have been sent to the user. The returned samples do not pass
through any additional filtering in the decimation process.
The PXIe-4480/4481 supports decimation with n = 1 to 15. The lowest supported rate is
1.333333 MS/s (20 MS/s ÷15). Note that NI-DAQmx always coerces the requested sample rate
up to the next sample rate that the hardware is capable of producing. For example, if the
requested rate is 1.26 MS/s, NI-DAQmx configures the PXIe-4480/4481 to use time domain
mode at 1.333333 MS/s.
Time Domain Mode Considerations
Comparison to Frequency Domain Mode
Because of the significantly different digital signal processing that occurs after Delta-Sigma
modulation, time domain mode has several key differences when compared to frequency domain
mode. The following list summarizes these differences.
•
Time domain mode is not inherently anti-aliased. Frequency domain mode acquisitions
always apply sharp FIR filtering to provide anti-alias rejection regardless of what sample
rate is used. In time domain mode, the filter programmed into the digital FIR filter always
operates at 20 MS/s regardless of the final rate returned to the user after decimation.
Therefore, it is possible for aliasing to occur when using the lower sample rates supported
by time domain mode.
•
The looser filtering used by time domain mode results in a noisier acquired signal. This is
to be expected, as time domain mode allows more bandwidth to come through. In addition,
some of the modulation noise that is fully rejected in frequency domain mode is only
partially rejected. As a result, the typical ENOB of time domain mode data is roughly
3 bits lower than frequency domain mode sampling at 1.25 MS/s.
•
Step responses measured in time domain mode are significantly less distorted than when
measured in frequency domain mode. The sharp anti-alias filtering done in frequency
domain mode is done with large FIR filters, and for that reason a sudden change in the input
produces noticeable ringing before and after the edge as the change propagates through the
FIR filtering. The magnitude of this ringing is significantly reduced in time domain mode
because of the smaller digital filter applied. Figure 3-7 shows a 0 V to 10 V step as
measured in frequency domain mode and in time domain mode. Note that the difference
in group delay has been compensated out so that the rising edges overlay.
