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U3-HV weird noise in pressure measurements

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#1 JustinLiang

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Posted 10 June 2013 - 11:19 AM

I am currently using LabJack U3-HV to measure the pressure difference between the intake and outtake of a filter. For the past two months I have had issues with noise in these measurements. They used to occur once in a while and would disappear like this:

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However, recently it has been occurring a lot:

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Here is the test setup (there are two pressure transducers and they both have the same noise, that is why I suspect it might be an issue with the DAQ:

Posted Image

Posted Image

#2 LabJack Support

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Posted 10 June 2013 - 03:26 PM

- Looks like you are using AIN1 and AIN3. Jumper AIN0 to GND and collect data from it at the same time.

- Collect data in raw volts, rather than scaled, so we can look at that.

- Can't see all the settings. Perhaps use <Alt-PrtSc> and get some jpegs of the entire window so we can look at everything.

- Does it make a difference if you only have 1 signal connected rather than both?

- Are these 4-20mA signals that you pass through a resistor to get voltage? Usually a low-voltage analog input works better than a high-voltage analog input. For example, a 120 ohm shunt resistor makes a 0.48 to 2.4 volt signal perfect for FIO4. See Section of the U3 User's Guide.

#3 TonyF

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Posted 27 June 2013 - 04:16 AM

When I set up a system to use pressure sensors with the LabJack, I put some voltage divider resistors and small smoothing caps on the board which held the sensors. Then connected the power in and signal out wires to the LabJack. The wires were no more than 40mm long. In this config I got a lot of noise similar to yours. Because signal and power share the same ground I thought that the problem was there. So I moved the resistors and caps to the LabJack and took the wires from there to the sensors. The noise disappeared. In your case it is strange that the problem has worsened over time, unless you have changed the wiring layout?

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