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2.6.3.3 - Signal Powered By the LabJack

A typical example of this type of signal is a 3-wire temperature sensor. The sensor has a power and ground wire that connect to Vs and GND on the LabJack, and then has a signal wire that simply connects to an AINx terminal.

Another variation is a 4-wire sensor where there are two signal wires (positive and negative) rather than one. If the negative signal is the same as power ground, or can be shorted ground, then the positive signal can be connected to AINx and a single-ended measurement can be made. A typical example where this does not work is a bridge type sensor providing the raw bridge output (strain gage bridge / pressure sensor / load cell … with no built-in amplifier) with non-isolated excitation voltage. In this case the signal voltage is the difference between the positive and negative signal, and the negative signal cannot be shorted to ground. An instrumentation amplifier is required to convert the differential signal to signal-ended, and probably also to amplify the signal. The U6 has an internal instrumentation amplifier, and thus can take the differential signal (AIN0-AIN1 for example) and also provide amplification.

2 comments

I have a temperature sensor which outputs 10 mV/F and the range of temperature to measure is ~ 60 to ~ 100 F (room temperature to human body temperature).  In order to measure the temperature more accurately, I could (1) connect the sensor to AINx and select the range of +/- 1v; or (2) put the sensor output to a amplifier (amplify with optional dc removal) so the signal fill the range +/-10 v, then connect it to AINx.   What would be the pros/cons of the two possible approach?  Thanks.  

Another way to look at that, is whether it is better to use the built-in amplifier (+/-1V range) or an external amplifier.  Pretty much the same thing.  I would recommend the internal amplifier.  We put a lot of time & development into the internal amplifier to make sure it works as specified, and you will have a hard time making an external amplifier perform that good.

One advantage to an external amplifier is speed.  If you are scanning multiple channels and need the fastest speeds possible (which you get on the +/-10V range), then you want to have an external amp on each channel.  In Section 3.1 and Section 3.2 you can see how the U6 is slower when you use the smaller ranges.