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Positive and Negative Current Measurements using same AIN?


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#1 JK3

JK3
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Posted 30 December 2014 - 07:55 PM

Hello,

 

I am working on a project using the LabJack U3-LV, and want to try to use the same AIN across a shunt resistor, to read both sinking and sourcing of voltages,and have unfortunately used all IO's except the one under discussion.

 

This configuration indeed yields negative and positive voltages, and I was hoping I could simply reverse the input channels accordingly, and separately for each function, i.e. positive = d.getAIN(7,31,1,0) and negative =  d.getAIN(31,7,1,0). Since channel 31 is not ground, this is problematic.

 

While I have read the manual and this forum extensively, I cannot come up with any solutions which do not involve more than one IO.

 

Thanks in advance for any suggestions!

 

Jim



#2 JK3

JK3
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Posted 05 January 2015 - 12:39 PM

I used an OPA330 and established a positive offset to get the positive voltage range I needed while using a single IO. :)



#3 LabJack Support

LabJack Support
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Posted 05 January 2015 - 02:06 PM

Good solution.  If someone wanted to do similar with a COTS product they could use the LJTick-InAmp from us.

 

 

Another consideration is that with a differential channel you can read an input down to about -0.3V.  Connect some other channel to GND with a small jumper, and then take a differential reading of your channel compared to that grounded channel.

 

 

We usually suggest hall-effect sensors for measuring current.  Many of these have an offset so a bipolar current produces a unipolar signal voltage.  Following is text from a support email about buying a hall-effect sensor:

 

For current, you need a current sensor.  I suggest a hall-effect
sensor.  A hall-effect sensor is isolated so you don't have to worry
about grounding or common-mode voltage issues.  You simply connect
VS/GND to the sensor to provide it power, and then connect the signal
wire to an analog input.

When I am looking for Hall Effect sensors  I start here at Digikey:

http://www.<span><sp...66573?k=current

First I go to the "Voltage - Supply" column and ctrl-select all ranges
that include 5.0V, then click "Apply Filters".  Then I go to the
"Current - Sensing" column and ctrl-select ranges that might work.  You
also might check the "In stock" box.  That usually leaves me with about
1 page of results.

When you select which ranges you want in the "Current - Sensing"
column, it is not always straightforward.  For example, if the max
current you want to measure is 10 amps, that does not necessarily mean
you should select a sensor rated for exactly 10 amps.  You have to
consider:

    - The signal output voltage of the sensor at different currents.
    - The signal input range of the LabJack.

For example, consider the SCD10PUN.  Following are its signal output
at different currents:

0A        0.3V
10A      2.3V
20A      4.3V

The U3-HV, U6, UE9, and T7, all have analog inputs that can measure up
to 5 or 10 volts, so the SCD10PUN is good for measuring up to 20 amps
with those devices.  The analog inputs on the U3-LV have an input
range of 0-2.4 or 0-3.6 volts, so it is good for 0-10.5 or 0-16.5 amps
with the SCD10PUN.




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