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Recently, I took computer architecture course, and got interested in analog electronics. At present moment, I can't afford good USB oscilloscope such as Agilent or Cleverscope (though, they are not that terribly expensive - $1500)

I am curious if USB Digital Acquisition Devices(DAQ) can be used as inexpensive temporary substitution for oscilloscope ? What would be drawbacks (apart from not having probes and having to record the data to PC) ?

Thanks !

Edit: some of the DAQ devices I have looked at:
http://www.ni.com/products/usb-6008/ (NI make an array of different USB DAQ) http://www.keithley.com/products/data/multifunction/usb/?mn=KUSB-3100 (though it looks like this DAQ is better suited for power electronics)

Some USB oscilloscopes I am interested in:

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I'm interested in the differences between a DAQ vs. an Oscilloscope, too, but what about investing into a bench-top scope? There are some pretty decent ones for around $400 or so, and they usually have a USB connection for hooking up to the PC. – helloworld922 Dec 25 '12 at 7:09
Probably 99% of all the available sub-$2K digital oscilloscopes are going to be only 8-bit. – Connor Wolf Dec 25 '12 at 8:34
The DAQ's yo are referring to have very limited bandwidth, barely enough to sample telephone quality audio. – jippie Dec 25 '12 at 10:06
The biggest disadvantage for USB scope's, in my opinion, is that they are often supported for the current Windows versions available. Once you want to upgrade you Windows to a newer release, the scope hardware is often rendered useless because of driver and post-processing software issues. – jippie Dec 25 '12 at 10:08
@newprint I am unfamiliar with Cleverscope, but most of the time this sort of devices have closed protocols and closed source software. Don't forget about the USB/.dll driver module here! Also writing your own software from scratch is not an easy task for many people. I've written software for a DVM, which is a lot simpler than a scope, and it took me several weeks to come to a useful tool (then again, I'm not much of a programmer). – jippie Dec 25 '12 at 10:44
up vote 7 down vote accepted

DAQ systems can make very functional low-speed oscilloscopes, with a number of caveats:

  • You're not going to get a very broad voltage range. Most of them will maybe do ±10V input range.
  • Probably won't support offset subtraction on the inputs, like scopes do.
  • DC-coupled only, unless you supply the series cap.
  • Inputs can be low(ish)-impedance (some may have buffer amps, on cheap ones the input may literally just connect to the ADC pin). Not the 1MΩ standard that scopes have.

  • Most importantly:

    • PC based oscilloscope interfaces suck
  • Also, it's likely a DAQ won't even have a traditional oscilloscope-like software tool. You may have to write your own.

Anyways, if you have a situation where you have fixed or low voltages, and don't mind doing a bunch of work on the PC end, a DAQ could be used as rather pokey oscilloscope.

They're really different tools, though, and while they do share some characteristics, they have very different intended uses, and this tends to show in their approach and the software design considerations.

It's also worth noting that most DAQ systems are designed for continuous, rather then triggered data acquisition. This means you're maximum sample rate is largely limited by the interface the DAQ uses.

For example, USB2 only has 480 Mbps(more like 400 Mbps real-world) bandwidth. As such the best sample rate that could ever be achieved would be 50 Msps(million samples per second) at 8 bits resolution, and very few implementations will even approach that. Somewhere in the range of 1-10 Msps at 8 or 16 bits is more realistic. Extracting all the available bandwidth from USB is very challenging.

Another consideration is what you're going to do with all the data. 1 Msps is a lot of data. If the 1 Msps stream is 16 bits, that's 2 Megabytes of data per second, or a gigabyte every 8 minutes. I don't know what you're intending to do with this pseudo-oscilloscope, but you can't just take samples willy-nilly, unless you're just displaying them and then immediately discarding them.

I've actually written a minimal real-time visualization tool for some IOtech-branded DAQ systems at work. It's kind of an oscilloscope. It works well, but I've also designed all the PCBs that interface with the DAQ system, so I could design them to work to the DAQ system's input specifications.

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Thank you for a long and detailed response. I have accepted your response as the answer. Somehow, I forgot all about the USB bandwidth. If Agilent claims that it's USB scope has sampling rate of up to 1GSa/s, how they manage to get all this date to PC ?? – newprint Dec 26 '12 at 2:04
@newprint the samples are all taken and stored on the device before they get transferred to the PC. – helloworld922 Dec 26 '12 at 3:23
@newprint - the critical thing is that the Agilent scope is non-continuous. It samples for a little while, and then spends a lot longer transfering that data to the PC, the duration of which it's not sampling the inputs. DAQ devices, pretty much universally, are going to be designed for continuous sampling. As such, you then need to worry about bandwidth concerns. – Connor Wolf Dec 26 '12 at 18:01
You can get DAQ devices that take more then 480 Mbps of data per second, but they're going to use a different interface, such as PCI-e, or cardbus. – Connor Wolf Dec 26 '12 at 18:01

Check ebay for low cost second hand equipment. HP and others brands has very long life and they are generally still very good even second hand.

The 2 DAQ models are lower in resolution and "high frequency" bandwidth than PC sound card. They can go down to 0 Hz DC whereas sound card typically cut off at 20Hz.

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Likely many DAQ ADC cards come with such software.

Some (not a lot) software use PC parallel port as very low speed logic analyzer. You can buy adaptor to make "parallel port' out of latest USB-only PC. However, this may limited the speed further.

Many software use PC sound card as oscilloscope/spectrum analyzer as well as signal generator. Apparently, these may more suit to EE engineering than computer architecture course. Allow on screen 'experiments' too. Like, http://www.qsl.net/dl4yhf/spectra1.html

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Personally, I wouldn't go there. You didn't specify a piece of equipment, but you would likely have speed issues. I would look at something like this. It's not very fast, but with a 25 MHz sampling frequency, you can easily look at 5 MHz signals, and theoretically up to 12.5 MHz. The transient recorder, bode plotter, and spectrum analyzer are nice features. The function generator is an added bonus, and most importantly, it doesn't cost $1500.

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Personally I am not really impressed with Velleman's software and its support over time. See also my comment at top of the page. – jippie Dec 25 '12 at 10:19

A scope is design to let you quickly and easilly probe a range of signals. This means a few key features.

  1. In most cases a high input impedance to minimise disturbence to the device under test (high end scopes often also have a low impedance mode for high frequency work)
  2. A systems to let you quickly and easilly change the settings. Physical knobs are much better for this than widgets on a computer screen.
  3. An input circuit that is tolerant of incorrect settings. You don't want your scope to blow up because you set it to the 1mV range and then put 10V on the input.
  4. Flexible triggering options to maximise your chances of getting a stable display.

Data aquisition devices on the other hand are more suited to longer term measurements. They typically have higher precision than scopes and are designed to sample continuously (whereas most scopes can only sample at their headline sample rate in bursts) but have a much simpler input stage that will often require external circuitry to get the signals into the right range. It's up to you to make sure that external circuitry is sufficiently tolerant for your application.

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