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Hardware Hacking Technology

A Workstation for Sensitive Experiments? 38

Posted by Cliff from the touchy-silicon dept.
etrgQUARK asks: "I am in charge of infrared spectrometers at our research center. One of the setups is used to measure the orientation of monolayers at the air/water interface, i.e. the signal we have to detect is very weak and noisy. We already have a great setup with quality components and electronics, except for one piece of hardware: the computer hooked up to acquire the data. How important is the computer in noise-sensitive data acquisition? What are your experiences? Do you have any suggestions on a workstation suitable for such tasks or is it a waste of money to use anything but the average computer system? Unfortunately, the software used is Windows-only."
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A Workstation for Sensitive Experiments? More

A Workstation for Sensitive Experiments?

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  • Data Logging (Score:3, Informative)

    by thegrassyknowl ( 762218 ) on Saturday December 03, 2005 @06:58PM (#14175333)
    You should look at expensive, custom data logging equipment for aquiring the information you need. When you have that you can shield it properly. The manufacturer's specs will show the sensitivity/noise/etc and you can select one that can actually record your signals with reasonable resolution.

    Don't run a PC anywhere near it if it's as sensitive as you say. PC's generate a lot of noise and they'll interfere with practically any sensitive measurements... take for example your TV. The TV isn't particularly sensitive but your PC can create noise on some of the channels.

    Just hook the data logger up to a PC after the experiment is complete.
    • I found the monitor to be the noisiest part of the computer. Switching to flat screen LCD almost completely removed noise from the computer setup. Most computers are very well shielded for static and that makes a decent Faraday cage.

  • Uh, by the time it gets to your PC it should be digital. So "noise" is not a worry unless you're planning on doing the A-to-D on the PC, in which case you need a psychiatrist, not /.

    --MarkusQ

  • Shouldn't this depend entirely on where the A-to-D conversion happens and how much EM interference the computer produces?

    As for the Windows dig, I can't see how Linux would result in less interference.

    • As for the Windows dig, I can't see how Linux would result in less interference.

      I think that the Windows reference was to dissuade someone from mentioning, for example, a gumstix [gumstix.org] wrapped in a roll of tinfoil (which was my first thought).

  • What sort of signal's are you recording exactly? Will they survive a long cable jaunt?

    Move box away from experiment. Problem solved.

    Are you worried about noise once the signals reach the machine internals (if they do)? If so, why would there be a internal data logger that can't log data correctly even on the market?

  • Amplify the signal you're looking for (Score:3, Informative)

    by hankwang ( 413283 ) * on Saturday December 03, 2005 @07:04PM (#14175370) Homepage
    The poster is a bit short on the details regarding what s/he considers to be 'good' quality data.

    With a 16 bits ADC you might get 14 bits effective resolution at the 10 V input range. The last 2 bits are often mostly noise even if you shortcut the input. Always use differential mode instead of single-ended such that noise from ground loops is eliminated. (It means it does an analog substraction of the signals on two inputs, rather than compare them to the common ground that may be noisy)

    If your noise requirements are much higher, then the best thing to do is to amplify the signal you're looking for before it goes into the ADC board. Use lock-in techniques if you can. For example if you're trying to see variations of 0.001 V on top of a 5 V signal, find a way to modulate the 0.001 V signal (e.g.chop the light source at 1000 Hz) and use a lock-in amplifier to measure the oscillating 0.001 V component and amplify it to some value that is easier to send to an ADC.

    • urm... dumb question, but isn't amping the signal a no-op in terms of SNR? by amplifying to (say) 20v and using a 24bit ADC you could get the 20 bits minimum to have a hope of finding the signal you're looking for, but you're still going to have to fight the SNR which, if anything, will have decreased (noise added by the amping, which in theory could be largely removed by amping the baseline signal and doing the differential stuff after the amping)

      unless (which is quite likely) i'm missing something...
      • isn't amping the signal a no-op in terms of SNR?

        The nice thing is that you can put the amplifier very close to the signal source such that the weak signal doesn't have to travel any long distance over which it can pick up RF interference.

        I'm not sure whether 24-bit ADCs really exist. Sure, professional audio recordings are done with 24 bits, but I doubt that you'll have more than 18 bits effective resolution in the ADC, and even then there aren't any microphones that can do much more than 85 dB SNR (14 b

        • thank you. unfortunately, i took a very cirtuitous route to signal processing (my formal training is as a computer scientist with a strong background in computer vision, which necessitated picking up image processing along the way. i'm discovering that my intuition for most things signal processing related isn't nearly as reliable as it is for other things)
      • Often, there are significant sources of noise within the reception or data capture device (such as the ADC). In these cases, it can be beneficial to amplify the input signal to that system.

        For example, suppose those 2 bits of noise are from the ADC unit itself. In this case, amplifying the input signal prior to reaching the ADC is beneficial. This especially holds true if the ADC is inside a computer system.

        Another good example is placement of a preamplifier before vs. after a long coaxial cable run. It

  • Backup (Score:2, Insightful)

    by Doctor Tesla ( 895547 )
    And of course, not entirely dependent on the "sensitivity experimentation" factor, but still: make sure to backup the results as much as possible. Try to have at least one external store of the data. After all, that could be one of the weakest links in your long chain of experimentation.
  • it's hard to really say. i imagine that you have a daq board with high precision A2D converters and sufficient buffering that so long as you can sustain the throughput you're not going to lose any data. (if not, you'd better fix this bit first)

    PC side, I'd suggest attention to the IO and memory subsystems if your data is arriving at a sufficiently high rate (of course, this requires your acquisition software to be written to take advantage of the hardware). that said, my suggestion would be: at least 1gb ra
  • You're seriously posing this question to a forum full of GNAA posts, The Giver links and "frost pist!" comments?!
  • Even though you're talking about something scientific, i'd like to point out that there is a very high demand for low-noise computers in the home-recording scene. (Low noise also equals low hum/shaking of the machine)

    Phil Rees computers are built to be extrimely quiet (and frankly i think your mouse makes more noise that one of these computers. www.philrees.co.uk [philrees.co.uk]

    I don't know if this helps.. probably better than most of these troll answers.
  • You didn't mention the kinds of noise you're interested in keep out, or what kind of data your sensors return. I will therefore assume, "all" noise and many kinds of sensors.

    The best approach to keeping something as electrically noisy as a PC from spoiling your results is to put it in another room and connect it only with radio or light. This also addresses the sound issue.

    If the bandwidth on your sensors is low enough for RS-232 serial data, then you're in luck, dozens of manufacturers sell simple in-l
    • Are you really reccomending a wireless transmitter as a solution to remove shot noise? Why not reccomend adding another elevator to the building? Or using only fluorescent lights.
      • Yeah, you weren't the only one thinking the radio suggestion was deleriously misinformed. Any networking here should be done over shielded twisted pair, honestly I'd suggest token ring. Fiber transceivers might make just as much EMI as anything else. If a 33.6k data rate is acceptable, modems will happily work over cabling with serious chokes on it. (And remember: Only ground one end of the shield.)

        Another poster mentioned CRTs as the major EM noise source. Seconded! Video cables are nasty too. If you want

  • Digital As Soon As Possible Please (Score:4, Interesting)

    by the eric conspiracy ( 20178 ) on Saturday December 03, 2005 @07:54PM (#14175596)

    PCs and long analog cable runs can definitely be a negative, especially if your signals are low level.

    My experiences with this sort of stuff is that you want to move the D/A converters as close to the experiment as possible and to use good instrumentation grade wiring with twisted pairs individually shielded plus a drain wire. If also sounds like your setup may be very sensitive to mechanical vibrations - if your noise source is mechanical nothing electronic will really fix the problem. You can filter stuff in the digital domain but you lose frequency response when you do.

    I've had very good luck with Analog Devices D/A stuff in the past; not particularly expensive and pretty good quality modules that you use in a distributed fashion to get into the digital domain as quickly as possible.

  • You should try talking to the people at National Instruments. You can have the best electronics setup in the world, but if you don't have an appropriate way of digitizing the data, your results will suffer. If all of your digitization is being done elsewhere and you don't have a ton of data to save, then you won't need a very good computer, but you also won't be able to use the computer as an active part of the experiment.

    You can digitize the data just fine at the computer, just amplify and buffer it befo
  • The faster the better and don't skimp on memory or coprocessors. Faraday cages are your friend. Also I dont' know if your setup could benefit but for my applications vibration tables significantly reduced noise.

    My qualifications? Single cell exocytotic measurements using 5 micron carbon fiber microelectrodes.

    And for the love of god do not attach the computer to the internet ever. Collect data and if you have to burn it to a CD and sneaker net it to machines that are.

  • One of the easiest thing that a noise-analysis hobbyist can do is configure the PC BIOS.

    1. Spread-Spectrum is a MUST enable.
    aka Clock Spread Spectrum
    aka Spread Spectrum Modulation

    Other EMI-reduction methods are:

    2. Older and slower PC have better noise level (may conflict with DAQ adapter requirements)

    3. Underclocking as much as possible on higher Ghz CPU. I'd prefer older and slower CPUs.

    4. A GOOD Metal Case. Aluminum isn't worth crap (Slashdot als

  • Computer should be a non-factor (Score:4, Insightful)

    by xtal ( 49134 ) on Saturday December 03, 2005 @10:08PM (#14176209)
    I design and deploy custom data logging solutions, signal conditioners, etc.

    Computer at the end of the chain should not be a consideration whatsoever. The system under monitoring should be completely seperate; by the time the computer is involved, it should be recieving a conditioned, pre-amplified, or digital data stream over a galvanically/optoisolated connection.

    If the introduction of a computer device causes a problem, there are other issues to consider.
  • I use similar equipment in my lab. CPU speed hasnt been much of an issue, but never never NEVER hook it up to the internet or a network.
  • preferablly connected via a low speed optical link (say rs-232 run over simple plastic fibre stuff nothing fancy needed)

  • I had to deal with this many years ago. The two biggest sources
    of interference were the CRT display and the switching power supply. That was pre-LCD so I had to live with the CRT but replacing the power supply with a linear regulator helped reduce noice significantly.

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