Intro and a question

Hi, my name is Alan and I have just received my Picoscope 2205A which I intend to use for my hobby which is mainly to do with audio electronics (both solid state and valves). This new scope replaces an old Pico ADC-212.
There seems to be a constant level of noise on the output even if the input is shorted. If I switch to the Spectrum mode, the noise is at a level of about -104dB and is at that level right up to 12.5MHz. Do you think I have a faulty device or is this normal (or am I doing something wrong)? I don't see how I can accurately measure THD+N with this inherent noise present, or can it be compensated for somehow. Incidentally, I have tried it on both my desktop pc and my laptop running on batteries with the same result.

That sounds normal.

Remember that the ADC212 has a vertical resolution of 12 bits, the 2205A only has a vertical resolution of 8 bits.

Thanks for the reply Martyn.
You mentioned the difference in vertical resolution between my old and new picoscopes. but surely that would only affect the amount of quantization error. It wouldn't cause it to display something that wasn't there.
I wondered if the 'noise' I was seeing was due to the usb 5v supply, so I tried using a usb hub which was powered by a linear 5v supply, but the 'noise' was exactly the same. So I must assume it is something inherent in the unit itself.
Incidentally, the software I used to use with my old scope used to have a measure for THD and THD+N, but I notice that this is not listed among the measurements available in the current software.

The Gaussian noise is present in the scope, and is used by the Resolution Enhancement feature in the time domain to gain extra vertical resolution. For audio work we recommend the 4262 as a replacement for the 212, it has a better noise floor.

The following measurements are available in the latest software

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Frequency at peak.
Amplitude at peak.
Average amplitude at peak.
Total power.
Total Harmonic Distortion (THD).
Total Harmonic Distortion plus Noise (THD+N).
Spurious-free Dynamic Range (SFDR).
Signal+Noise+Distortion to Signal+Noise Ratio (SINAD).
Signal to Noise Ratio (SNR).
Intermodulation Distortion (IMD).

which version are you running ?

Hi again, thanks for your previous reply.
I am running software version 6.8.11.20 which I believe is the latest version.
I can't see those measurement options you mentioned from the Measurements tab/Add Measurement/Select Type of Measurement. I assume they are not available for my scope.
Overall, I am very pleased with the 2205A, I think it represents excellent value for money, (and is an amount I can justify spending!). I am particularly pleased with the signal generator facility, which is a really worthwhile instrument in its own right!

If you are in Spectrum Mode the measurement options should change to the ones I listed, they do on the 2204a I have connected.