Noise floor levels

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m-sonntag
Posts: 2
Joined: Thu Sep 11, 2014 2:55 pm

Noise floor levels

Post by m-sonntag » Thu Sep 11, 2014 5:29 pm

I recently bought a red pitaya and just quickly hooked it up in order to see what noise levels I get from it.
As I can see in the specs the ADC has a resolution of 14 bits. With a range of -1V to +1V that gives a quantization of approx. 122 uV.

So far so good. Now I hooked up a probe to one of the pitayas inputs and let it float (unconnected to anything) expecting to measure 50 Hz disturbances from the environment. The graph showed this:
Image
Image

I get noise with a standard deviation of around 1.4 mV.


Then I tried to short the probe to ground
Image
Image

which suddenly gives me a noisier signal and even some spikes (see the picture). The standard deviation is now around 2.3 mV.
Of course I can use the moving average filter but then high-frequency parts of a signal will get filtered out which I do not desire.

Are these normal noise levels for a Red Pitaya? If not what could be the cause?

m-sonntag
Posts: 2
Joined: Thu Sep 11, 2014 2:55 pm

Re: Noise floor levels

Post by m-sonntag » Tue Sep 16, 2014 11:51 am

So I tried shielding, using different power sources for the RP, removing any other electrical devices from the vincinity pf the RP. As far as my tests go I can say that about 4 bits of the ADC are lost due to noise. Is this normal or is my RP broken?

hmaarrfk
Posts: 26
Joined: Fri Jun 27, 2014 3:19 pm

Re: Noise floor levels

Post by hmaarrfk » Sun Sep 21, 2014 12:51 am

If you want to actually test the noise level of the RP, try simply connecting a 50 ohm load to the ADC.
Even with that, I have noticed about the same noise levels has you have. Maybe even more :S.

I suspect that 4 bits of noise is pretty good. Honestly, most scopes have 8 bit ADC because anymore more than that requires serious calibration.

I think the parts themselves quote 80 dB SNR, but that is under ideal power supply situations.

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