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U3 and LJTick-InAmp


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#1 ZimacLabs

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Posted 12 July 2011 - 12:20 PM

Hi, I misinterpreted the spec for a Omega PX137 pressure transducer (see attached). I failed to see that the output is indeed 0 to 90 mV DIFFERENTIAL, and not referenced to ground. Anyways, to solve the issue I ordered some LJTick-InAmp modules to attach to the U3 read the signals correctly. The problem is that our current software platform is scaled to read 0 to 100 mV on those particular inputs. Is the output level from the LJTick-InAmp low enough to be compatible or should we introduce some scaling in the software to handle this? Thanks, Franco

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#2 LabJack Support

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Posted 12 July 2011 - 09:57 PM

So it is a raw bridge sensor that provides 90mV at Vexc=12V and rated pressure. General info about bridge sensors:

https://forums.labja...?showtopic=4376

What are you using for excitation voltage? 12V will be no good for the LJTIA. Rather you should use the 2.5V reference on the LJTIA:

http://labjack.com/s...heet/appendix-d

Using 2.5V excitation, you will get 18.75 mV at rated pressure, so from the signal range tables you would use Voffset=0.4 and Gain=x51:

http://labjack.com/s...heet/appendix-a

You will want to measure the actual offset of the applicable channel on your LJTIA, not just assume 0.40 volts. If the input is 0 to 18.75 mV, the output will be about 0.4 to 1.35625 volts, so sounds like you will need to change your software.

#3 ZimacLabs

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Posted 13 July 2011 - 08:27 AM

Changing the software to introduce some offset and scaling is not a problem, as we're using Labview VI's to read the U3. I spoke to our developer and he doesn't foresee any difficulty there. What I'm worried about is using 2.5V instead of 12V to power the sensor. Yes, I am currently exciting it with 12 VDC. I was hoping to simply connect the output of the sensor as is (using pins 3 and 4) to the IN+ and IN- of the LJTIA and work from there with scale and offset. I'll have to test the sensor at 2.5V and see what it gives as an output. I'll let you know. Thanks, Franco

#4 LabJack Support

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Posted 13 July 2011 - 08:41 AM

It looks like the sensor is just a raw bridge, and thus you can excite it with anything up to the max of 16 volts. Confirm with the manufacturer. With lower excitation, you got proportionately lower output, but the 2.5 volt reference is perfect for the LJTIA and will usually outperform external supplies even if they are higher voltage.

If you use Vexc=12V and connect the supply minus to U3 GND, you will have a common-mode voltage of 6 volts. From the LJTIA signal-range tables, and the AD623 calculator that is linked, you can see that is no good:

http://labjack.com/s...heet/appendix-a

If you know the 12V supply is isolated from the LJTIA, you can not connect the supply minus to GND but rather connect signal minus to GND (also connected to IN-). This gets rid of the large common-mode voltage.

#5 ZimacLabs

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Posted 09 August 2012 - 12:49 PM

Hi, LJ Support Sorry to "zombie" my own thread, but I just want to report that this has worked out beautifully. We are getting very repeatable, stable measurements for vacuum level. It's difficult to read in the photo, but it's just shy of 28 in Hg on the gauge (the large numbers are in kPa) Thanks again for your great support. Frank

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