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