Increasing current in 6238 inputs

[Loadcell]

Hello,

I am using the 1042Q chasis with 8106 Controller and 6238 DAQ.  I have hooked up the output of a sonic sensor to the DAQ (pins 4 and 22).  When I run a vi to measure the current, I find that the current remains at a steady expected value for about a minute, but then slowly rises to the maximum.  If I try and read the channels with the sonic sensor power turned off, or even the 6238 cable unplugged unplugged, I notice that the input current quickly rises above the measurement maximum of 20mA.  Any idea on what could be causing this?  Since this happens with the channels themselves, even though the DAQ is not connected to anything, could this be an internal grounding issue?

Any tips would be appreciated, thanks.

[samantham]

Hello,

What configuration method from the NI 6238/6239 User Manual (page 4-3) are you using for the Analog Input.  I am concerned that you have a ground loop when the sensor is connected.  In addition, what type of sensor are you using?  Do you have a brand and model number?

As far as what the 6238 is measuring when no signal is supplied, I will start looking into this question.

[Loadcell]

    Hello,

you are right, I had a grounding issue.  I'm using an ultra sonic sensor with current output, and I wired the + output to pin #5 and the ground to pin #23 (AI 3 + and -) however I needed to put the AI ground in connection to AI3- and now it works fine.  Before I had AI3- going to the power supply ground, so I had a power supply ground and a DAQ ground and they were not the same.  I was trying to use method 1 in the manual, but am now using method 2, which works just fine.

I do have a question abotu the "Shunt resistor value" in the LabView examples for reading in current using this board .  Does this actually do anything?  I have used large and small values and my input current reading never changes.  This is supposed to be used for devices that cannot drive a large current, right?  Mine can be connected to 500Ohms, so I assume the DAQ will have no trouble since it has a 100Ohm input impedence.

Thanks!

[samantham]

Hello,

The "Shunt Resistor Value on the Front Panel of the Current Input examples actually related to the "External Shunt Resistor Value input for the DAQmx Create Channel (Current Input).vi  This value only needs to be filled out when using an external resistor.  The 6238 has an internal resistor, thus the "Shunt Resistor Value" is never processed.  For the "Shunt Resistor Location" input, I would recommend selecting default, which will determine if the device does or does not have an internal shunt resistor.

As long as you are supplying a current with the +/- 20 mA range and staying under the overvoltage protection specifications you should be okay.  I have linked the NI 6238/6239 Specifications for you convenience.