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Low input voltages

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Low input voltages

Postby Natan » Mon Aug 28, 2006 11:23 am

I am considering purchasing a Pico 3000 oscilloscope, but my input voltages will be extremely low, less than 3V. Therefore, it is important that the A/D resolution be high. Is it possible to spread the 8 bits of the A/D converter over a range of 0-3 Volts? Can I customize the imput range?

In any case, what is the maximum imput voltage possible? I don't think I saw it on the specifications page.
Natan
 

Postby Natan » Mon Aug 28, 2006 11:35 am

I would like to ask the same about the 3424 scope
Natan
 

Postby ed800uk » Tue Aug 29, 2006 9:29 am

Natan,

While we're waiting for the experts, here's a word from a user.

I hope I understand your question.

(Note firstly that some of the 3000 series scopes use 12-bit quantisation, so depending on your needs, they might be suitable already.)

The issue is that you don't want to "waste" headroom on the chosen input range?

If your signal is from zero to +3 volts, and you selected a range -5 volts to +5 volts, you would be "wasting" 70% of the range.

There might be two simple things that you could try. The first would improve to a wastage of 50%. The second (less likely to succeed) might reduce wastage to zero.

Firstly.

You might be able to regain part of the range by attenuating your signal so it just fits into a lower range, namely -2 volts to +2 volts.

Effectively you would be making a special probe.

Depending on your signal you might be able to simply include a half megohm low noise resistor. This would be in series with the specified one megohm input impedance of the scope, attenuating by one third your signal voltage, and reducing your maximum excursion to +2 volts.

You could then use the Pico feature of "custom ranges" so that the software displays correctly your signal voltage.

Secondly.

You could try to regain all the remaining wastage, if you could float the ground voltage of the scope together with its attached PC. You should only do this if you understand its implications. Also you would be able to use more than one channel only if all channels shared a common zero volts from your signals.

Still using the half megohm resistance probe, you could change the range on the scope: select -1 volt to +1 volt.

Your signal from zero to +3 volts would then be seen by the scope as -1 volt to +1 volt.

Once again the marvellous "custom ranges" could be used to clarify the display.

No wastage at all. But something might get seriously damaged unless the scope/PC were battery operated and isolated from any "ground" in common with your source.

This is only my opinion and I welcome any weakness in the analysis.

EDIT: I had another think about this and I don't think the second part would work. You would have to ensure that the "ground" of the scope/PC was exactly 1.5 volts above your 0 volt level. It would be subject to a lot of electrical noise. You might make it work by AC coupling your signal (not internally in the scope but with a suitable capacitor on the probe).

I don't think you would ever be successful with other than a single channel.

Unless you go for a complicated "active" probe, I think you have to live with 50% waste (1 bit) due to the disused negative-going part of the input that the scope sees.

Regards to all,
Ed
ed800uk
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Postby Sarah » Thu Aug 31, 2006 12:00 pm

Hi

I have to say that Ed's post pretty much covers it all! I would endorse the first suggestion of using the potential divider to fit into the 2V range. The second idea carries some damage risk with it so I would hesitate to use that.

Let me know if I can be of any further help.

Best Regards
Sarah
 

Postby Natan » Tue Sep 12, 2006 8:35 am

My system needs four channels, therefore I am interested in the 3424 scope. I plan on connecting the 3424 to a laptop while riding in a vehicle and using the data logger. What will the sample rate be?

Also, the ride may be a little rough. What shocks is the scope designed to withstand?
Natan
 

Postby Sarah » Wed Sep 13, 2006 11:20 am

Hi Natan

Thank you for your post.

The sample rate will depend on your settings however using the scope in this scenario will not affect the sampling rate you can achieve.

The unit has not been designed specifically to withstand shocks and has not been tested as such so I am unable to advise here. It also depends on what you mean by "a little rough" :)

Best Regards
Sarah
 


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