Technical Notes: Debugging Noisy Data Acquisition Systems

This article discusses how to trouble-shoot fluctuating measurements.

Some monitoring systems, such as pH, are very sensitive and prone to interference. Small current or voltage surges can cause large fluctuations in readings.

Here are some simple tests that you can follow to determine the cause of measurement problems. We've used the Microlink 751 multi-function USB unit as an example but many of the tips also apply to other manufacturer's systems. Work through the tests one-by-one, eliminating sources of error as you go.

  1. Check that the Microlink is working properly. First remove other factors in the system that might be causing the problem, then check the Microlink itself.
    1. Remove any power signal conditioner. This provides surge protection and noise filtering. It should remove interference from other equipment but a faulty one could cause its own problems.
    2. Move the Microlink to another location. If you are using it in a laboratory for example, take it home.
    3. Connect the Microlink to a different laptop running on battery power, not on mains power.
    4. Now you can test whether the Microlink is the source of the noisy, fluctuating, data. Connect together the first positive input pin, the first negative input pin and the 0 V input pin. With the Microlink 751 these inputs are on pins 20, 1 and 19. You could use paperclips to connect the three pins. Leave all other inputs unconnected.
      You have now shorted out the Microlink: if it is working correctly it will produce a reading of zero. If not the fault lies with the Microlink and you should contact Technical Support and send the unit back. If the reading is zero then the problem is very unlikely to be caused by the Microlink and you can go on to step 2.
  2. Once you have eliminated the DAQ unit as the source of the fault, test the power supply to the computer. Plug the laptop into the mains - the reading should still be zero.
  3. Reconnect any power signal conditioner. Again, check that the reading is zero.
  4. Test the sensors one-by-one. For example, place a pH probe in a known solution and see if the reading is as expected. Keeping the signal wires short and far away from electrical machinery helps reduce noise. You may also need to clean your electrodes.
  5. Move everything back to the original location. If the readings become erratic go through the tests again.
  6. Disconnect from the laptop and connect to the original computer. Again, if you now get noisy readings repeat the tests.
  7. Finally make your measurements in a real situation.

Discovering at which stage the problem occurs will let you identify the source and take remedial action.

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