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2.6.3.8 - Floating/Unconnected Inputs

The reading from a floating (no external connection) analog input channel can be tough to predict and is likely to vary with sample timing and adjacent sampled channels.  Keep in mind that a floating channel is not at 0 volts, but rather is at an undefined voltage.  In order to see 0 volts, a 0 volt signal (such as GND) should be connected to the input.

Some data acquisition devices use a resistor, from the input to ground, to bias an unconnected input to read 0.  This is often just for “cosmetic” reasons so that the input reads close to 0 with floating inputs, and a reason not to do that is that this resistor can degrade the input impedance of the analog input.

In a situation where it is desired that a floating channel read a particular voltage, say to detect a broken wire, a resistor (pull-down or pull-up) can be placed from the AINx screw terminal to the desired voltage (GND, VS, DACx, …).  A 100 kΩ resistor should pull the analog input readings to within 50 mV of any desired voltage, but obviously degrades the input impedance to 100 kΩ. For the specific case of pulling a floating channel to 0 volts, a 1 MΩ resistor to GND can typically be used to provide analog input readings of less than 50 mV. This information is for a low-voltage analog input channel on a U3.

Note that the four high-voltage channels on the U3-HV do sit at a predictable 1.4 volts. You can use a pull-down or pull-up resistor with the high-voltage inputs, but because their input impedance is lower the resistor must be lower (~1k might be typical) and thus the signal is going to have to drive substantial current.

5 comments

Why does a differential reading between two channels give a different value than the difference of two single-ended readings? I have found discrepancies over 0.5V in magnitude, but they are not always the same. Sometimes the two methods give almost exactly the same result; sometimes not.

Thanks.

The most common mistake would be connecting 2 signals to AIN+/AIN- that are not referred to U3 ground in any way, and thus you just have 2 floating signals.  See the Differential Analog Inputs app note.

Hi

I am having an issue to read input signal from the Labjack T7 Pro. It should be mentioned that Labjack is connected to the network via Ethernet and kipling has detected the device. Basically I am trying to use the Python file 'single_ain.py' to read the unconnected AIN0 signal. My intention is to make sure that my software can read the input signal. I am using Eclipse to run 'single_ain.py' file. When I run this file in eclipse it shows an error, which is as follows:

Traceback (most recent call last):

  File "/home/mushtafiz/Python_LJM/Examples/AIN/single_ain.py", line 9, in <module>

    handle = ljm.open(ljm.constants.dtANY, ljm.constants.ctANY, "ANY")

  File "/usr/local/lib/python2.7/dist-packages/labjack/ljm/ljm.py", line 270, in open

    raise LJMError(error)

labjack.ljm.ljm.LJMError: LJM library error code 1227 LJME_DEVICE_NOT_FOUND

 

Can you please let me know what is wrong there ?

 

thanx

 

Make sure other programs, such as Kipling, that are using your T7 over Ethernet are closed and retry running the example. The T7 can only have one Ethernet connection to it at a time.

I already fixed the problem. You are right, I were running kipling and Eclipse at the same time and it was conflicting.

Thanks for your quick reply.