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crosstalk on analog inputs


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#1 cashout

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Posted 21 December 2013 - 08:05 PM

I am not an electronics wiz and am stumped why analog input signals follow each other on my U6 set-up:   Keyence IL1000 distance scanner (-5 to +5 VDC output analog signal with 100 ohm impendence) is connected to AIN0 on U6.  The reference GND from the IL1000 is connected to GND on U6 with 100 ohm resistor in sseries.  A 24VDC output from a small stepper motor is connected to AIN1 with a 22K resistor in series to pull the voltage down to 10VDC.  The U6 is isolated and both analog inputs are powered externally.  The IL1000 signal and REF GND wire to the U6 is shielded twisted pair.  The motor signal uses a separate shielded cable.  Both the IL1000 and stepper motor are powered from the same external DC power supply.

 

Using LJStreamUD, the data from AIN0 (distance scanner) looks good.  However, the other analog channels follow AIN0 each with a small offset. AIN1 from the motor likewise follows AIN0 when low, but does properly go to 10V when high.

 

Connecting the external power supply ground directly to U6 ground flattens the signal on AIN0 to almost nothing; i.e. the proper data on AIN0 is only shown correctly when the IL1000 reference ground is connected to U6 GND.

 

During  streaming using LJStreamUD, a laptop PC is connected to the U6 via USB with the PC running only on battery with it's power supply disconnected.

 

We do not have a real electronics guru on staff but are truely at a loss to our problem.  I've tried 10K resistors between AIN0 and GND and AIN1 and GND which helps flatten the signals but does not correct the following problem.



#2 LabJack Support

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Posted 21 December 2013 - 09:16 PM

So the IL1000 and stepper motor are both powered by the same 24VDC supply.

 

Sounds like the +/-5V signal from the IL1000 is referred to a REFGND  and based on your results REFGND might not be the same as the supply ground, and yet it also seems like REFGND is not isolated from supply ground.

 

A couple questions:

 

Before connecting any of this to the U6, use a DMM to measure from the IL1000 signal REFGND to supply ground.  That is, can you measure any voltage difference between REFGND and supply ground?

 

I don't understand your connection of the stepper voltage to AIN1.  You need a voltage divider, not just a 22k in series.  The series resistor technique is for a digital input where you use the internal diode to clamp the signal:

 

http://labjack.com/s...s-guide/2.8.1.1

http://labjack.com/s...s-guide/2.6.3.6



#3 cashout

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Posted 22 December 2013 - 10:05 AM

With the LJ disconnected: I measure a 5.7V voltage difference between IL1000 REFGND and system GND.  The IL1000 is set to "-5V to +5V" vs an alternate setting of "0V-5V".  The motor input has been disconnected from the LJ in all but FIG4 to isolate the IL1000.

Attached Figures 1-4 show examples of collected data. 

 

In summary, accurate data is shown from the IL1000 with REFGND connected to LJ GND or SGND (FIG1).  Substituting system GND for REFGND results in no signal from the distance scanner.  Connecting both REFGND and system GND to LJ GND produces a flattened signal from the IL1000; i.e. the data is not accurate (FIG3).

 

Could the problem be related to the IL1000 settings?

Attached Thumbnails

  • FIG1.jpg
  • FIG2.jpg
  • FIG3.jpg
  • FIG4.jpg


#4 LabJack Support

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Posted 23 December 2013 - 12:32 PM

Your tests seem to confirm that REFGND is not the same as supply ground on the IL1000, and also the 2 are not isolated.

 

I don't think it matters if you set the IL1000 to +/-5V or 0-5V output.  I would just stick with the +/-5V output.

 

Trying doing a differential measurement.  Run 3 wires from your system to the U6:

 

    - signal to AIN0

    - REFGND to AIN1

   - supply ground to GND

 

Now take the differential reading AIN0-AIN1 and see if that is correct.

 

Also note the single ended reading from AIN1.  Does it stay around 5.7 volts over time and as the differential reading varies from -5 to +5?

 

From Appendix A, you want to make sure the voltage on each AIN versus GND stays in the range +/-11.5V.  If AIN1 is sitting at 5.7V, and the differential signal is 5V, that means AIN0 is at 10.7V which is OK.  As long as AIN1 (REFGND versus supply ground) stays at 6.5V or less your readings should be good.

 

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



#5 cashout

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Posted 28 December 2013 - 11:09 AM

Thanks for the support.  I implemented the changes for a differential measurement as outlined above,  The signal is now virtually perfect.




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