Valid Sampling Rates for NI DSA, CM C Series and SC Express Devices

Updated Jul 15, 2019

Reported In

Hardware

  • NI-9237
  • NI-9234
  • NI-9227
  • NI-9230
  • NI-9231
  • NI-9232
  • NI-9250
  • NI-9218
  • NI-9219
  • PXI-4461
  • PXI-4462
  • PXIe-4463
  • PXIe-4464
  • PXIe-4480
  • PXIe-4492
  • PXIe-4497
  • PXIe-4499
  • USB-4431
  • USB-4432
  • NI-9229
  • NI-9235
  • NI-9236

Issue Details

I have a device with a Delta-Sigma ADC like a NI DSA, C Series or SC Express device and I have noticed the following scenarios.
  • When I set a constant sampling rate on my NI Dynamic Signal Acquisition (DSA) or CM C Series, that rate appears to change, or my data is sampled at a different rate than what I requested.
  • My data seem unexpected based on the constant sampling rate I have set. Am I using an invalid sampling rate on my NI DSA device?

Solution

Devices with Delta-Sigma ADCs have different timing properties.

Delta-Sigma C Series devices have discrete sampling rates. Requesting a different rate will result in the module defaulting to the discrete rate closest to the requested sampling rate. The valid samples rate for these devices will be given by a formula that can vary with the module. The formula can be found in the module datasheet's Data Rates section. For example, NI 9234 Datasheet and NI 9237 Datasheet show the valid sample rates are give by
 
where fs is the sampling rate, fM the internal timebase on the device and a number between 1 and 31.

SC Express and DSA devices have a finer timing resolution given by the Sample Rate Range and Sample Rate Resolution specifications. For example, the PXIe-4339 sample rates an be set between 1 S/s and 25.6 kS/s in increments of 181.9 µS/s.

It is possible to read the actually used sample rate using the DAQmx Timing Property Node:
 

Always refer to your Delta-Sigma device's documentation for accurate information about its valid sampling rates.

Additional Information

Delta-Sigma C Series, DSA and SC Express devices have discrete sample rates because of Delta-Sigma Analog to Digital Converter (ADC). These require oversampling, which means they require their own Sample Clock Timebase. These timebases can only be divided down by discrete numbers.

Taking again the NI 9234 and NI 9237 as an example, while both devices use the same formula, because the internal timebases are different (13.1072 MHz for the NI 9234 and 12.8 MHz for the NI 9237), the valid sample rates will differ. If you set a sample rate of 2.4 kS/s on each device, the NI 9234 sample rate will be coerced to 2.438 kS/s and the NI 9237's to 2.5 kS/s (both with n = 21) because those are the next values that result from the formula.

If you use an NI-9237 in the same task as an NI-9234, both will run at the 9234's rates by default. This is because DAQmx synchronizes multiple modules by automatically routing the faster master timebase to the other modules. However, you can force the task to use any of the available timebases by manually setting the Sample Clock Timebase Source property using the DAQmx Timing Property Node:
 

For NI-446x, NI-447x, and NI-449x devices, the respective formulae are stated in NI Dynamic Signal Acquisition User Manual starting on page 2-24.

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