Can DAQ devices be used as Oscilloscopes?

Question

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:
http://www.cleverscope.com/products/CS320A
http://www.home.agilent.com/en/pc-1418982/usb-modular-oscilloscope?nid=-34492.0&cc=US&lc=eng

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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 with a wide range of voltage settings 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.

Answer 5

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.