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Sampling Rates - Confused Novice

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Sampling Rates - Confused Novice

Postby MrZebra » Thu Oct 22, 2009 2:30 pm

Hello,

I'm considering buying a 220X series scope, but I'm a bit confused about the sampling rates - maybe you could clarify it for me?

For reference, I'm planning on measuring digital signals up to around 1MHz, and analog signals up to around 100kHz.

The 'Equivalent Time Sampling' (ETS) I think I understand - it's used when you have a repetitive signal, the scope always triggers at the same part of the waveform, and then takes samples at different offsets from that trigger point to build up a better picture of the waveform. This won't be useful to me because my data signals will come in bursts, not repetitive.

The 'streaming sampling rate' is what should be useful to me. This is 1MS/sec on all 220X models. What confuses me here is the "2M sample limit". Does this mean that it stops 2M samples after triggering? Or that it will display continuously on the screen, but if I press 'pause', I will only be able to "scroll back" 2M samples? If it's streaming the data, then surely these samples are held in the PC's memory which is likely to be a few GB, in that case why such a small limit?

The 'block mode': if I understand this correctly, it reads a number of samples and then stops reading to transmit the data, leaving gaps in your data - I can't really see how this is useful? It's just for looking at data immediately following a trigger?

The 'maximum real time sampling rate'. This is neither the streaming rate or the ETS rate but somewhere in the middle - is this the rate for block-mode measurements? Is this used with triggers, so that if I set up a trigger using say the 2203, then it would read 8000 samples at 40MS/s, then stop?


One last question - this time about grounding. I have read the warning about the ground potential, but I don't really understand what I can do about it. If my circuit is powered by some power supply other than the PC (eg a bench power supply or external switched-mode power supply), how do I safely connect the scope? I must admit to ignoring this warning on other devices because I just don't know what can be done about it!

Thanks for any help you can provide,

--- MrZebra
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Re: Sampling Rates - Confused Novice

Postby Erik » Fri Oct 23, 2009 6:54 am

The reason for a block mode is to get the sampling rate up above what can be transferred across the USB bus in realtime. If you look at our faster devices like the PS5000, it samples at 1GS/s.

There is no way we can transfer that across the USB (480Mb/s = theoretical 60MB/s) which in real terms gives us 30MB/s for user data after bit stuffing and other overheads.

Block mode will write to the devices memory at the full sampling rate of the product and then we can read it out on the slower USB bus. Triggers are essential to get the data you are interested in. This mode works very much like an old analogue oscilloscope.
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Re: Sampling Rates - Confused Novice

Postby MrZebra » Fri Oct 23, 2009 8:46 am

Hi, thanks for your reply, but it didn't answer all my questions.

1) The spec lists a 2M sample limit in "continuous streaming mode" (1MS/s). Does this mean that it stops 2M samples after triggering? Or that it will display continuously on the screen, but if I press 'pause', I will only be able to "scroll back" 2M samples?

2) I have read the warning about the ground potential, but I don't really understand what I can do about it. If my circuit is powered by some power supply other than the PC (eg a bench power supply or external switched-mode power supply), how do I safely connect the scope? I must admit to ignoring this warning on other devices because I just don't know what can be done about it!

Thanks again,

--- MrZebra
MrZebra
 

Re: Sampling Rates - Confused Novice

Postby ziko » Wed Nov 04, 2009 8:29 pm

Hi the 2MS is a limit in Picoscope 6, you can have up to 2MS points for each screen capture or waveform buffer. So when in streaming mode as you increase the time base the sample rate will reduce, since the maximum samples in capped at 2 million sample points for each waveform buffer.

In terms of the warning:

You will short circuit whatever you connect your ground clip to. So if for example you connected it to something that is floating then you should be ok. If you clipped your ground clip of say the ground terminal of a power supply that is not floating than you will be ok. If however you connected your ground clip to a positive i/p to a power supply or source that is not floating you may damage the oscilloscope and possibly your DUT or have undesirable effects.

I hope this helps.

Kind regards
Ziko

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