PXIe 6368 noise floor

[ASIM_BHATTI]

Hi all

I am using PXIe 6368 card and am trying to calculate self noise of each channel in this card. For this purpose, i select a differential channel, set input range to +/-5 V, short both differential inputs and performed following steps:

1. Collect 'x' no of samples of voltage signal
2. Perform FFT of size 'x' on collected samples
3. Divide each complex output of FFT by 'x' 
4. Find the absolute of each complex output of FFT
5. Multiply the output of above step by 1.414 to get Vrms against each frequency bin
10. Take log10 and multiply by 20 of each output in above step 
11. Add 10xlog10(x/2) in each output of step above to compensate FFT gain against noise

The result is self noise floor graph of a channel in terms of dBVrms Vs frequency.I got average noise floor around -68 dBVrms across frequency band.

Now, according to specifications of this card at page-3, it has random noise of 134 uvrms for +/-5V range scale (which in my assumption is self noise/channel of this system) which results in  -77dBVrms approximately.

If my method of measurement and calculations are correct and also the assumption that this card has self noise spectrum of -77dBVrms for a frequency band, there is difference of around 9 dB between measured value and theoretical value.

Kindly guide me whether i am doing right calculations and if i am right, what may possible reasons for this difference of 9 dB?

Thanks

[Conner.A]

There are a number of reasons you may be experiencing noise in your signal. Your calculations seem to be correct, and represent the best case scenario for this device with no external noise. This indicates to me that there is external noise to the system. Check out this document on noise and how to set up your system to minimize it:

Field Wiring and Noise Considerations for Analog Signals

http://www.ni.com/product-documentation/3344/en/