Problem with acquisition PWM signal with cDAQ chassis and NI-9219

[Jiri Uraj]

Hello Together,

maybe I missed something, but I have build testbench, which is controlled by PC and signals are acquired by cDAQ chassis and AI cards. Now, I have to add testing of black box, which has at output PWM signal in levels: low: ca 4,3 V, high: ca 12 V, frequency 30 Hz.  Only one free channel is at card 9219, universal analog input card 24-bit. I built structure in Labview with I have an experience with acquiring and computing PWM at another multifunction hardware like USB DAQ 6009 or 6216. At these HW everything works fine, but at this cards it doesn't work.

But at NI-9219 analog input I have still the similar voltage ca. 9,0 V. If I check black box by osciloscope in laboratory I can see fine PWM signal. I think I missed something about NI 9219 card and it is not good for this application, but I don't have another choice. Could anyone more experience or any solution in this case?

Setting of card: rate: 1000 s, samples: 1000, connection: Differential

Thanks for any help.

Regards

Jiri Uraj.

[Stefo]

Hi Jiri,

If you try to measure analog input (cDAQ with 9219) sampled on 1000S/s, than the thing you might missed is sample rate supported by NI 9219 - it is 100 S/s. If you use it in cDAQ, you might not see the error saying that you exceeded maximum rate, but instead it will repeat the sample multiple times. For more information about why you could see this, see following documents:

NI CompactDAQ Technology

NI CompactDAQ under the Hood – Technologies That Drive USB Performance

With kind regards / S pozdravom,

Martin Stefik