Please forgive my newness to LabJack, but I am seeing some odd behavior with floating (unconnected) channels that raises some concerns about the ADC.
Just got my T7 and got installed and got the streaming sample setup, compiled, and running in .NET (C#)
I run the example and look at the first buffer of data and there are voltage reported that are outside of the allowed signal range (+/- 10V), -10.5, -10.3, etc. If I tie a channel to the ground it works as expected and comes in a 0V. I don't have one of our machines (shock dyno reading displacement, force, and temperature) available so I can't test it as
The general point is I've never seen an ADC report voltages outside of the particular signal range, +/- 10V in this case.
So what is going on? My experience with other manufacturers National Instruments, Measurement Computing, and a variety of motion control platforms have me conditioned to see this as something being wrong.
I'm sure I missing something. I'd like to understand what it is.
So far the experience with the board has been great development-wise, I've been able to quickly build the component needed for our program and this will may LabJack an easy item to integrate with our machines.
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The behavior you are seeing is expected. There is an instrumentation amplifier at the analog inputs and the amp inputs are not tied to GND with a resistor, so when the channels are floating the amp bias current can pull the inputs out of the common mode range (towards one of the power rails):
https://labjack.com/support/app-notes/floatingunconnected-inputs
Your readings are also within the actual input range; when we put ±10V it is just a shorthand/approximation of the range. Our typical range specs can be found in the datasheet appendix:
https://labjack.com/support/datasheets/t-series/appendix-a-3-2-1-t7-gene...
Expect the drift to the rails, but never seen a rail outside the valid units range before. Make sense now.
Am I correct in reading that the possible allowed voltages are +/- 11.5 V?
So for the T7 we loose some resolution (v/int) , 23V/65536 = 0.0003509521 V/int versus 20v/65536 = 0.0003051758 V/int
Appreciate the time you have taken to explain this to me.
The actual, typical span for the ADC is what is listed as the "Typical Input Range". The ±11.5V is the valid common mode range of the instrumentation amplifier in front of the AIN. This is also somewhat of an approximation. We have more in-depth common mode range information in the following section:
https://labjack.com/support/datasheets/t-series/appendix-a-3-2-3-t7-sign...