NI cDAQ 9178 with 2 card NI 9234 and 2 card NI 9215 for data logging using signal express

[Abhijit204]

I am using 2 card NI 9234 24 bit DAQ card for 6 microphones and one card NI 9215 16 bit DAQ card for hotwire probes on cDAQ 9178 chassis board and NI signal express for data logging. However, i wanted to sample hotwire data at 32768 HZ for NI 9215 but NI 9234 does not sample at this sampling rate using internal master timebase (either 25.6 ks/S or 51.2 ks/S). My questions are in two fold.

1) If I sample all the signal at 32768, will the NI 9234 synchronize with Ni 9215 internal master timebase  and be able to sample at 32768?

2) Can I use NI 9215 16 bit DAQ card to sample microphone data? Will it cause any drifting or error? 

I look forward to your reply. Thanks

[tievans]

Hi there,

To answer your questions I will need a bit more information from you:

1) How is your hardware set up?

• What connectors are you using?
• Are you using any external circuitry?
• Why are you using the specific modules for these measurements

2) How are you timing your measurements?

• In Q1 in your post do you mean using the internal timebase on your cDAQ chassis device?
•  Why are you not using a derived clock?

I have also attached some information that may be useful to you:

NI microphones guide: http://www.ni.com/white-paper/14349/en/

NI Synchronisation guide: http://www.ni.com/white-paper/11369/en/

Synchronization in LabVIEW: http://www.ni.com/white-paper/11466/en/

Please let me know if any of these are helpful. I will be able to provide more specific information once you have answered my questions.

Kind regards,

Timothy Evans

[Abhijit204]

HI, Timothy

Thank you for your reply.

1) How is your hardware set up?

• What connectors are you using?

Ans- I am using normal BNC connectors to connect DAQ card channels to microphones amplifiers channel.

• Are you using any external circuitry?

Ans-  No, I am not using any external circuitry. I am using LAB VIEW Signal express built-in portal for the data logging. 

• Why are you using the specific modules for these measurements?

Ans- These microphones require 24-bit DAQ but I have only two 24-bit DAQ and two 16-bit DAQ but I require 9 channel for 6 microphones, 2 hotwire probes and 1 signal generator data logging. Therefore, I have no other option but to use two 24-bit and one 16-bit DAQ.

Please note that I am using Signal express for the data logging application and not the LabView sub-vi programming. 

2) How are you timing your measurements?

• In Q1 in your post do you mean using the internal timebase on your cDAQ chassis device?

No, I meant internal timebase of those 24-bit and 16-bit DAQ modules. According to the internal timebase of 24-bit NI 9234 card, they sample only at either discrete sampling frequency (e.g.25.6ks/s or 51.2 ks/s..etc) but 16-bit DAQ NI 9215 can sample at any sampling frequency. However, I want to sample at 2^15 sampling frequency. What I worry is if 24-bit DAQ NI 9234 will follow same sampling frequency if I sample at 2^15 = 32768 S/s?

•  Why are you not using a derived clock?

I am suing Signal express, I am unsure if I can use a derived clock in that signal express window.

[tievans]

Hi there,

Thank you for replying to my questions. I have a couple of follow up questions for you:

1) Is there a reason you can't use the internal clock on the cDAQ chassis?

2) Do you have any specific concerns for error, either for the software or the hardware on the device?

Concerning synchronisation on your devices I would like to make the following points:

• The use of two separate clocks can lead to drift error and is something that should generally be avoided when synchronisation is needed in measurements. This is even when the devices are set to sample at the same sampling rates. You can try to minimise this error by using a single clock or by using some of the timing methods detailed in our DAQ course which you should have access to through online training (triggering etc.).
• The devices you are using are not deterministic hence there will be jitter induced in your sampling times. This can be a problem when you are using two different clocks for your measurements.

I hope this information is useful to you. For more information please look through our website and the training provided via you NI account.