Differential analog in


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I am connecting some thermocouples to my LabJack U6 AIN's on a CB37. I am evaluating how I should connect it to the CB37 terminals. I have already tried the single-ended approach, and am trying the differential approach now. I have it all wired up to the AINx terminals. Question is: how do I get DAQFactory to know its a differential input and react accordingly? Thanks.

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  • 2 years later...

Put the desired negative channel in the Quick Note / Special / OPC column

This isn't quite the whole answer. I get a subtly different result using this technique versus creating a conversion that subtracts the two channels -- each the default A - D (single-ended) configuration. Treating the two channels as separate inputs then subtracting them by way of a conversion gets the correct answer -- zero (plus or minus the 0.6mV quantization) when the LabJack terminals are physically connected to the same signal. Using the QuickNote technique results in an average value of 4.0 to 4.6 mV.

I suspect that the difference is in the conversion of ADC codes to calibrated volts. If that conversion has an offset as well as a scaling then the offset is applied improperly to a channel configured in psuedo-differential mode. With the Labjack configured to return code values that have been subtracted already, only the scale factor should be applied.

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See the following app note about differential inputs:


Remove all connections from the U6 except for USB. Connect +tc to AIN0, connect -tc to AIN1, and connect a jumper from AIN1 to GND.

Start by testing with LJLogUD. Leave the 1st row as Channels 0/199 and the 2nd row as 1/199, and set the 3rd row to 0/1 (differential AIN0-AIN1). Set the range of all 3 rows to +/-100mV (BIPP1V). Set resolution to 0 (which equates to 8 on a U6).

Now look at the voltages. The 1st and 3rd row should read quite similar, and the 2nd row should read close to 0.

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