Slashdot Mirror
A Workstation for Sensitive Experiments?
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."
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 drink to make other people interesting!
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.
Avantslash: low-bandwidth mobile slashdot.
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.
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.
Classic case of engineering not listening to customer requests. The question stated that it was for equipment to monitor the air/water interface. Microwaves no matter how weak will impart some measure of rotational and vibrational energy to the water molecules. Thus increasing noise in the already noisy system. The sensors may be microwave proof but the analyte is not.
I've hit Karma 50 and gotten a Score:5, Troll... I win!
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.
..don't panic