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Minimising base noise read by PicoScope3224

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Minimising base noise read by PicoScope3224

Postby alexis » Wed Jan 13, 2010 4:10 pm

An average 38mV p-p noise is measured by PicoScope 6 when set to +-5V. The p-p noise will decrease if the range is changed, but i need to measure the noise present on the output of a DC-DC supply which is in the range of 3-4.5v.

Reading the forum i see the mains from the laptop may inject noise into the oscilloscope, it could also come from RF captured by the probes. I have performed the following test:

Laptop on battery With the picoscope not connected to anything: 38mV p-p
Laptop on mains With the picoscope not connected to anything: 38mV p-p
Having the probe and ground connected to each other: 38mV p-p
Shorting signal and gnd with a wire at the picoscope bnc connector: 38mV p-p

Is this expected?
Is there a way i could measure more accurately my mV noise even with the 5V offset from ground?

PS. i have some saved waveform but see no method of uploading them atm.
alexis
 

Re: Minimising base noise read by PicoScope3224

Postby alexis » Wed Jan 13, 2010 4:16 pm

here are two waveforms with a short wire shorting the picoscope bnc output
Attachments
20100113-0001 laptop mains disconnected.png
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20100113-0001 laptop mains connected.png
(20.49 KiB) Downloaded 5 times
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Re: Minimising base noise read by PicoScope3224

Postby gruntman » Wed Jan 13, 2010 5:38 pm

Alexis,

The 38mV noise that you are seeing on the +/- 5V scale is the inevitable noise built into all 8 bit resolution scopes. At 10V pk-pk voltage range you can figure the lowest step the ADC can display by dividing 10 by 256 (the number of steps in 8 bits). If you step up to a 12 bit scope this would be decreased down to 2.4mV (10/4096).

Since you are measuring a DC signal and simply wish to measure the noise, turn on AC coupling and then you can lower your input range down to the mV scale...

Regards,

Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com
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Re: Minimising base noise read by PicoScope3224

Postby alan » Thu Jan 14, 2010 5:21 pm

Alexis,

In the channel options you can also set low pass filters and resolution enhancement. These can be useful methods of reducing noise at the expense of reducing bandwidth.

Regards
Alan
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Re: Minimising base noise read by PicoScope3224

Postby alexis » Fri Jan 15, 2010 12:13 pm

Thanks for the reply,
Switching to AC coupling was indeed what i was looking for since i can now have a window of +-20mv and the base noise will now be under 1mv level. This lets me use the oscilloscope nicely!

I do want to point out that the PicoScope3224 is a 12bit oscilloscope so at 5000mv/4095steps ~1mv, so im still not sure what is causing the 38mV pp.

Maybe the USB power is still being very noisy even when powered from the laptop battery..

I do agree the filtering or the resolution enhancer would both filter out the noise but im trying to measure the noise in my circuit atm.
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Re: Minimising base noise read by PicoScope3224

Postby angelsix » Wed Feb 03, 2010 9:30 pm

I'm having very similar problem here topic5643.html
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