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Why do I have a measuring inaccuracy?


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3 replies to this topic

#1 Tm!

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Posted 23 July 2012 - 05:18 AM

Hello, I'm trying to read out the voltage of a photodiode (1-2um) with the LabJack U12. In addition to the diode I have an amplifier to amplify the signal coming from the diode. In order to test the signal recording with the labjack I did a measurement with an alternative data acquisition logger. Unfortunately I measured a slightly different value compared to the measurment afterwards with the LabJack (difference of 500mW). And even with an oscillograph I could notice an inaccuracy compared to the LabJack. Can someone explain to me where the inaccuracy comes from? Thanks.

#2 LabJack Support

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Posted 23 July 2012 - 04:33 PM

So you connect your signal to AI0 and GND, and then connect the DMM to AI0 and GND at the same time? What voltage do you see on the DMM and the U12 when connect to both at the same time? How much noise do you see?

#3 Tm!

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Posted 24 July 2012 - 01:18 AM

I don't know what you mean with DMM, sorry. However, I first connected the diode to AI0 and GND at the LabJack and after I have measured some values I disconnected the diode from the LabJack and connected it to an alternative data acquisition logger and recorded some values under the same conditions. In doing so, I noticed a little difference between the values displayed.

#4 LabJack Support

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Posted 24 July 2012 - 09:27 AM

A DMM is a digital multimeter, or a voltmeter:

http://en.wikipedia....wiki/Multimeter

You can use your other DAQ device for testing.

Connect the amp output (not the raw photodiode signal) to your DAQ device and see what it reads. Now connect to AI0 and GND on the U12 at the same time. When you connect the amp signal to the U12, what the voltage reported on the other DAQ device and see if it changes? Does LJlogger report the same voltage as the other DAQ device? What voltages do you see, and how much noise?


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