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Externally Powered Signals (App Note)

Externally powered signals derive power from batteries, external power supplies, or perhaps AC mains wall power.  The most common example is a 3-wire sensor with DC voltage supply:  Vsupply, Common, and Signal.  Vsupply and Common connect to some DC power supply.  Signal connects to AINx on the LabJack, and you usually also need a connection from Common to GND on the LabJack.  Those simple connections are true for almost every case, but more details about ground complications follow.

For example, there is a "black box" sensor with a wire coming out that is defined as a 0-5 volt analog signal and a second wire labeled as ground. The signal is known to have 0-5 volts compared to the ground wire, but the complication is what is the voltage of the box ground compared to the LabJack ground.

If the box is known to be electrically isolated from the LabJack, the box ground can simply be connected to the LabJack's GND. An example would be if the box was powered by an internal battery, and does not have any wires besides the signal and ground which are connected to AINx and GND on the LabJack.

If the box ground is known to be the same as the LabJack GND, then perhaps only the one signal wire needs to be connected to the LabJack, but it generally does not hurt to go ahead and connect the ground wire to the LabJack GND with a 100 Ω resistor.   You definitely do not want to connect the grounds without a resistor as you are creating ground loops that add a lot of noise to your system.

If little is known about the box ground, a DMM can be used to measure the voltage of the box's ground compared to the LabJack's GND. As long as an extreme voltage is not measured, it is generally OK to connect the box's ground to the LabJack's GND, but again, it is a good idea to put in a 100 Ω series resistor to prevent large currents from flowing on the ground.  Use a small wattage resistor (typically 1/8 or 1/4 watt) so that it will break if too much current does flow which may prevent your LabJack from being damaged in the process.  The only current that should flow on the ground wire is the return of the analog input bias current, which is on the order of micro-amps (for the U3 & UE9) to nano-amps (for the U6).

The SGND terminals available on LabJack devices can be used instead of GND for externally powered signals.  When connecting an externally powered signal to this terminal a series resistor is not needed as SGND is fused to prevent over-current, however a resistor will eliminate confusion that can be caused if the fuse is tripping and resetting so it is still a good idea.

In general, if there is uncertainty, a good approach is to use a DMM to measure the voltage on each signal/ground wire without any connections to the LabJack.  If no large voltages are noted, connect the ground to the LabJack's SGND with a 100 Ω series resistor.  Then again use the DMM to measure the voltage of each signal wire before connecting to the LabJack.  

Another good general rule is to use the minimum number of ground connections.  For instance, if connecting 8 sensors powered by the same external supply, or otherwise referred to the same external ground, only a single ground connection is needed to the LabJack.  Perhaps the ground leads from the 8 sensors would be twisted together, and then a single wire would be connected to a 100 Ω resistor which is the connected to the LabJack's ground.  

4 comments

I have a DC power supply which provides a 5V signal for monitoring the output voltage and current. They have separate (+) leads and a common (-) lead, which does not appear to be the supply's ground (though I am not sure).

The labjack (u6) is hooked up to a laptop. The laptop is plugged into the wall.

AIN0 (and that screw terminal's ground) is used to hook up a thermocouple (which works fine as is).

I think this means I need to use AIN2 and 3 and AIN4 and 5 in differential mode for the current and voltage signals? But I need to still somehow provide a ground reference? Would a 10Kohm resistor to ground from the negative terminal of both work?

How do I hook this up?

Likely you can simply connect each signal+ to an analog input and connect both signal- to GND, but the connection you described is extra-conservative and should work well without risk of any ground loops.

I'm using a pair of Labjack U6's to monitor 12 strain gages (each monitors 6) using differential inputs on AIN0 - 11. I am also monitoring the excitation voltage powered externally (10V DC source), again using differential input (V+ on AIN12 and V- on AIN13). I am grounding the U6's together using the GND screw terminal and attaching to the ground on the DC source. 

The issue I am having is that the voltage readings I am getting are really noisy. The excitation voltage I get from the reading jumps around from between 9.2 and 10.1, but when I use a multimeter to check it, it reads at a steady 10.0.

Any suggestions?

It is possible that the signal is actually noisy and the DMM is averaging it.  If you start removing other connections does it change anything?  Eventually, if you remove all connections and just have 1 U6 with USB connected, and connect the DC supply to AINx and GND, does it read better?  I would respond on the forum or through email to [email protected] so we can continue with the details.