Low Frequency Alias Rejection with NI Dynamic Signal Acquisition (DSA) Modules

Updated Jul 3, 2018

Reported In

Hardware

  • PCI-4461
  • PXI-4461
  • PXI-4462
  • PCI-4462
  • PXIe-4464
  • PCI-4474
  • PXI-4472
  • PCI-4472
  • PCI-4472B
  • PXI-4472B
  • PXIe-4499
  • PXI-4498
  • PXI-4496
  • PXI-4495
  • PXIe-4497

Issue Details

What is Low Frequency Alias Rejection? Which revisions of my DSA modules support this feature?

Solution

Presently, the following devices support low frequency alias rejection: 
  • NI 4461/4462 and NI PXIe-4464*
  • NI PXI-447x devices revision H and later
  • NI PCI-447x devices revision F and later**
  • NI 449x devices
* The NI PXIe-4464 always has this option enabled.
** The NI PCI-4474 is not intended to support this feature.

The USB and C Series DSA devices do not support Enhanced Low Frequency Alias Rejection. These devices include the NI USB-4331/2 and the NI 9233/4.

This feature is enabled by default for the 4461 and 4462 modules but is disabled for the 447x and 449x modules for rates above 1 kS/s.

This feature is controlled by the AI.EnhancedAliasRejectionEnable property of the DAQmx Channel Property Node (Property»Analog Input»General Properties»Advanced»Enhanced Alias Rejection Enable).

Low frequency alias rejection is recommended when the sampling rate is less than 25.6KS/s. Due to the nature of a delta sigma converter, noise centered around certain multiples of the sampling rate pass through the digital filtering process. The bandwidth of noise that can pass through the digital filter is always one Fs wide and centered around the oversampled Fs multiple.  

To prevent noise from passing through the digital filter, an analog filter with a fixed cutoff frequency above the maximum sampling rate is placed before the analog to digital conversion occurs. Normally, Fsmultiples are well above the analog lowpass filter's cutoff frequency and are thus removed by the analog filter before they reach the digital filter.
For example, when we acquire greater than 102.4kS/s, the first oversampled Fs multiple on the PXI-4462 is:
32*Fs (see the specifications sheet for your particular DSA card to find the oversample rate)
=32 * 102.4kS/s
= 3.2768 MHz

Since 3.2768 MHz is well above the analog filter's cutoff frequency, any noise between 3.1744 MHz and 3.3792 MHz (3.2768 MHz±102,400/2) is removed by the analog filter before reaching the digital filter.  In other words, none of the noise that could theoretically pass through digital filter ever reaches the digital filter.

At rates below 25.6kS/s, the analog lowpass filter does not filter noise in the oversampled Fs multiples because they are below the fixed analog cutoff frequency.  

For example, when we acquire at 1 kS/s with the PXI-4462, the first oversampled Fs is 128 kS/s, which is below the analog filter's cutoff frequency. As a result, any noise between 127 kS/s and 129 kS/s passes straight through the analog AND digital filters and is digitized as an aliased signal.

To prevent this, we can enable Enhanced Alias Rejection, which forces the actual sampling rate to never fall below 25.6kS/s.  If the user defines a sampling rate below 25.6kS/s, the driver will automatically set the sampling rate to an integer multiple of the desired sampling rate as follows:
 
Sampling RateDecimation Factor
100 S/s ≤ fs ≤ 200 S/s256*
200 S/s ≤ fs ≤ 400 S/s128*
400 S/s ≤ fs ≤ 800 S/s64*
800 S/s ≤ fs ≤ 1.6 kS/s32*
1.6 kS/s ≤ fs ≤ 3.2 S/s16
3.2 kS/s ≤ fs ≤ 6.4 kS/s8
6.4 kS/s ≤ fs ≤ 12.8 kS/s4
12.8 kS/s ≤ fs ≤ 25.6 kS/s2
 
*These decimation factors apply only to NI449x devices.

Once the signal is digitized, the resulting waveform is decimated by the same integer multiple so that only every Xth sample is returned.  Thus, the user-defined sample rate is obtained post-acquisition.  The benefit of Enhanced Alias Rejection at sample rates below 25.6KS/s is that no noise contained in Fsmultiples reaches the delta sigma converter, which means there are no aliased images.

Additional Information

For more information on the theory behind Low Frequency Alias Rejection, see the Dynamic Signal Acquisition User Manual, linked below.

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