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uV level resolution?

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uV level resolution?

Postby Guest » Wed Nov 04, 2009 9:38 am

I couldnt' help but notice that none of your so called PC oscilloscopes could detect signals in the uV range. The typical range low range was +/- 50mV. Most real scopes can go clear down to +/-100uV. Now I'm looking to replace my old analog scope (which can go down to +/- 100uV) with digital scope with the same specs as my analog scope. Now my college (the University of Washington) has some nice digital scopes that go down to +/- 100uV. But when I check the models online, they go for at least $2000 (the same as some of your more expensive Picoscopes, but even your more expensive scopes don't get better than +/- 10mV). I'm looking for a way to get the SAME performance as a full standalone digital scope but at a fraction of the cost. The first thing that comes to mind is a computer based scope, because with a computer you allready have the hardware for processing data, and can get software that emulates the controlpanel and display of a regular digital scope. And ofcourse your monitor is the display in place of the built-in LCD screen on a digital scope. So the external hardware needed for a PC based scope really is just an A/D converter with a USB link for control and data transfer. And it SHOULD be able to perform the same as a complete standalone digital scope. And it should be able to achive the same results at a small fraction of the price of a standalone scope. I wanted to buy one of your Picoscopes because of how cheap they were. But I decided not to buy any because they don't perform as well as any digital scope I know of, not even as good as my old analog scope. Until you improve the performance of your cheapest good model (the $261 one with the AWG feature, that I actually was considering buying) to make it have a resolution down to at least +/- 100uV on the smallest scale, I'm not buying your product.
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Re: uV level resolution?

Postby Erik » Wed Nov 04, 2009 2:38 pm

When you are comparing ranges and resolution bear in mind that.

Our 50 mV range is the same as:
    10 mV/div
    390 uV/count (with 8 bit resolution, 2000/3000/5000 series)
    24.4 uV/count (with 12 bit resolution 3000/4000 series)

Our 20 mV range is the same as:
    4 mV/div
    156 uV/count (with 8 bit resolution, 2000/3000/5000 series)
    9.8 uV/count (with 12 bit resolution 3000/4000 series)

Which other digital scopes where you looking at that can do down to +-100uV?
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Re: uV level resolution?

Postby Guest » Wed Nov 04, 2009 9:23 pm

[quote="Erik"]When you are comparing ranges and resolution bear in mind that.

Our 50 mV range is the same as:
[list]10 mV/div[/list]
[list]390 uV/count (with 8 bit resolution, 2000/3000/5000 series)[/list]
[list]24.4 uV/count (with 12 bit resolution 3000/4000 series)[/list]

Our 20 mV range is the same as:
[list]4 mV/div[/list]
[list]156 uV/count (with 8 bit resolution, 2000/3000/5000 series)[/list]
[list]9.8 uV/count (with 12 bit resolution 3000/4000 series)[/list]

Which other digital scopes where you looking at that can do down to +-100uV?[/quote]

I think the ones at the UW (which are Tektronix TDS-2004b) can go down to +/- 100uV full scale , or about 10uV/division. I believe my analog scope has the same sensitivity. As my analog scope no longer works (horizontal deflection is dead), I've decided that my replacement should be digital (not gonna waste money to fix the old one or by another analog scope). However, I also don't plan to spend over $2000 on one. But I also don't plan to get a scope with less sensitivity than my (now dead) analog scope.
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Re: uV level resolution?

Postby gruntman » Wed Nov 04, 2009 10:25 pm

Hello,

The Tektronix TDS-2004b units feature a vertical sensitivity of 2mV/Div. This is with 8 divisions so a total screen of 16mV. Note however that the bandwidth of the unit goes down to 20 MHz when you choose this range.

The majority of analog and digital scopes have a lowest sensitivity level of around 1mV-50mV/div. Some are significantly higher such as 50V/div. 10uV/div, while not impossible, would be very rare.

While voltage ranges are a important factor to figure in when researching a new oscilloscope, resolution, buffer size, bandwidth, sampling rate, ease of use etc are all also equally important.

It depends on your application which unit would be the best for you. For example, my applications typically do not involve measuring voltage as low as 10uV's, so I don't need to have a range that low. If I did need it, I would probably find a way to amplify the signal.

Regards,

Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com
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Re: uV level resolution?

Postby Guest » Thu Nov 05, 2009 5:45 am

[quote="gruntman"]Hello,

The Tektronix TDS-2004b units feature a vertical sensitivity of 2mV/Div. This is with 8 divisions so a total screen of 16mV. Note however that the bandwidth of the unit goes down to 20 MHz when you choose this range.

The majority of analog and digital scopes have a lowest sensitivity level of around 1mV-50mV/div. Some are significantly higher such as 50V/div. 10uV/div, while not impossible, would be very rare.

While voltage ranges are a important factor to figure in when researching a new oscilloscope, resolution, buffer size, bandwidth, sampling rate, ease of use etc are all also equally important.

It depends on your application which unit would be the best for you. For example, my applications typically do not involve measuring voltage as low as 10uV's, so I don't need to have a range that low. If I did need it, I would probably find a way to amplify the signal.

Regards,

Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com[/quote]


Problem with amplification of the signal (lets say 10uV becomes 1mV, or a gain of 100), is that the scope will report 1mV, even though the signal I'm trying to measure is only 10uV. So my readout is inacurate. But then the scope software would have to compensate. Even more, such signal pre-amp wouldn't have a NIST calibration if I made one myself, so 10uV might ACTUALLY get me 1.1mV or 0.9mV, instead of the desired 1mV. And it might drift over time from giving 1.0V to 1.1V because I (as an individule rather than a company) wouldn't have the best parts. Lastly, the simplest amplifier to make is with an op-amp chip. But due to finite slew rate these are only linear up to about 20kHz and that ruins specs of my oscilloscope then. You mentioned the TDS-2004b does get better resolutions of less uV/pixel than Picoscope, and when in this mode limits its frequency response. Do you think you could implement that in you Picoscopes?
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Re: uV level resolution?

Postby gruntman » Thu Nov 05, 2009 6:06 am

Hello,

Every additional piece of equipment that you add in between the scope and the actual signal is going to degrade the accuracy you get in some form or the other. There are differential preamplifiers available to purchase which can be NIST certified (and really anything can be NIST certified essentially, it just takes a spec sheet and a cal house that will work with you), but these can be quite pricey. The issue with changing the preamplifier down to the uV ranges is the cost. With readings in the uV range the slightest RF noise, electrical shock etc can cause a significant reading change on the scope. The cost and time spent on EMC testing would be quite high. The majority of our customers are happy with the ranges we currently have, if we get enough requests for lower ranges however it would be something that we could look into further.

Regards,

Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com
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