Noise floor of PCI-6281

[NoiseAnalyzer]

I am trying to analyze the noise of PCI 6281.

When I FFT the analog input with 50 kOhm termination without any voltage, the intensity noise seems ~ -60 dBc/Hz.

Is this right?

[Frank-L]

Hello,

I am trying to find out some more information to assist you. Is there a problem with the card and you did this as a test, or are you just wondering what the spec is so you can use this information in your application. Currently, we do not spec out the noise floor for these PCI devices. This Knowledgebase provides some information on how to find out the noise floor on a different device, but may prove useful to you in your application.

[NoiseAnalyzer]

There is nothing wrong with PCI card. 

I was trying to use PCI card to analyze the noise spectrum of my instruments, but I found that PCI has the high noise floor of ~ -100 dBm/Hz at input range of +- 10V with 50 kOhm termination. 

I am just wondering what the spec is. I know my PCI card has the random noise of 70 uVrms, but I want to know the noise spectrum provided by NI.

And is there a way to activate onboard low-pass filter using the Matlab?

[Frank-L]

As far as the noise spectrum specifications, we do not generally have the noise spectrum as a specification in our NI 628x Specifications for our DAQ devices. Like you were saying earlier, the only noise specification provided for NI-DAQmx devices is within the specification manual. For DSA devices which require this information as part of its typical application, we do provide specifications for these devices. Since we do not have these certain specifications internally or externally, we cannot answer this question adequately and any values you received could very well be the typical values for these devices..

For low pass filtering, you should take a look at An Introduction to NI-DAQmx Tools for Data Acquisition with The Mathworks, Inc. MATLAB® Software. This is a good resource on how to program NI-DAQmx using Mathworks, Inc MATLAB® Software and may be able to provide you some additional information. Coding in MATLAB is similar to using C based code samples and in the C reference help there are functions    to enable the Anti-Aliasing filter. Get/Set/Reset AI_Lowpass_Enable is the function used in C programming and should be a good start to programming in MATLAB.

[NoiseAnalyzer]

Thank you Frank, it is really helpful.

But I do not see any examples on activating onboard low-pass filter using the matlab.

Can you help me with this?

[Frank-L]

Noise Analyzer,

I have done some more research and it seems we do not provide that in depth of examples. The introduction that I showed in the previous email is the extent of our MATLAB examples using NI-DAQmx, and we do not have a specific MATLAB code for enabling the Low-Pass Filter. The closest thing that we can provide for MATLAB is the examples for C API which use similar function calling methods. Listed below is the exact C code for enabling the Low Pass filter on these devices as found in the C reference help of NI-DAQmx. For more information regarding coding NI-DAQmx C functions in MATLAB it would be best to contact MATLAB support as we do not have any information on how to do this other than the examples provided in the "NI-DAQmx tools" library found on the tutorial I posted earlier. Thanks for your understanding, and have a great day!

int32 __CFUNC DAQmxSetAILowpassEnable(TaskHandle taskHandle, const char channel[], bool32 data); 

Frank