Sampling rate on two of three NI Daq irregular (NI Daqmx)

[joe77777]

Hi,

I am acquiring signals from 3 different Daq cards. The third one (voltage) the sampling rate is totally fine and as I set a rate of 100. 

The other two (RTD and thermocouples) have a very irregular sampling rate and changing it doesn't seem to solve the problem. I have attached a file and a picture of the output. 

The instrumentation is not faulty as I have tested it with the NI max and also with the DAQ assistant. Both cases the sampling rate seems to be regular and as set (100).

Thanks!

[ZYOng]

What are the model numbers of the modules you are using? Some temperature input modules have a sample rate of below 100S/s.

Besides, using the High-Resolution Relative Seconds VI or Tick Count is not the correct way to determine the actual sampling rate of the acquisition. It is just the iteration rate for the loop containing the DAQmx Read VI. Read from the property Sample Clock >> Rate under the DAQmx Timing instead. See the shipping example Help >> Find Examples... >> Hardware Input and Output >> DAQmx >> Analog Input >> Voltage - Continuous Input.vi

[joe77777]

I'm using the NI 9210, NI 9217 and the NI 9215.

I checked at the sample clock and that's where i put 100 and continuus samples in, or am I misunderstanding?

[ZYOng]

Here are the maximum sampling rates for each module:

NI 9210: 14 S/s

NI 9217: 400 S/s

NI 9215: 100 kS/s/ch

So basically NI-9210 is slowing down the while loop containing DAQmx Read VI. When you request a sampling rate of 100 Hz, you don't necessarily get what you request. In the case of NI 9210, only 14 S/s are given. The actual sample rate can be read from the property node.

[ZYOng]

If you want to read all channels at 100 Hz despite NI-9210 can only sample at 14 Hz, you can use the 1D Waveform instance of the DAQmx Read VI. All channels in the waveform data type will have the same sampling rate. See Using the Waveform Data Type in LabVIEW and Accuracy of the Waveform Timestamp Returned by NI-DAQmx to learn more about this data type. The data from the slower NI-9210 will be resampled such that it is duplicated until a new value is updated.

[joe77777]

Hi,

thanks for your answer. 

I'm using the 1D waveform now and I've used the DAQmx timing to see what sampling rate I'm using. Apparently the 100Hz as wished for. It is true that the data is getting resampled and duplicated. But the NI 9210 is not even doing 14 samples per second but only around 3 samples per second. The same is happening with the DAQ 9217 and this is supposed to have an even higher resolution than the DAQ 9215, right?

Could that be a problem of maybe not displaying a sufficient resolution on the data with not enough digits after the comma?

Thanks!

[ZYOng]

NI 9210: 14 S/s

NI 9217: 400 S/s

NI 9215: 100 kS/s/ch

If you see the maximum sampling rate of the modules, only NI-9215 has a /ch at the end. This indicates that NI-9210 and NI-9217 have multiplexed sampling, in which all channels share a single ADC. If you use all 4 channels of NI-9210, the maximum sampling rate per channel is 14Hz/4Ch = 3Hz.

Multiplexed Versus Simultaneous Sampling