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2.6.3.9 - Signal Voltages Near Ground [U3 Datasheet]

The nominal input range of a low-voltage single-ended analog input is 0-2.44 volts.  So the nominal minimum voltage is 0.0 volts, but the variation in that minimum can be about +/-40 mV, and thus the actual minimum voltage could be 0.04 volts.

This is not an offset error, but just a minimum limit.  Assume the minimum limit of your U3 happens to be 10 mV.  If you apply a voltage of 0.02 volts it will read 0.02 volts.  If you apply a voltage of 0.01 volts it will read 0.01 volts.  If you apply a voltage less than 0.01 volts, however, it will still read the minimum limit of 0.01 volts in this case.

One impact of this, is that a short to GND is usually not a good test for noise and accuracy.  We often use a 1.5 volt battery for simple tests.

If performance all the way to 0.0 is needed, use a differential reading (which is pseudobipolar).  Connect some other channel to GND with a small jumper, and then take a differential reading of your channel compared to that grounded channel.

The nominal input range of a high-voltage single-ended analog input is +/-10 volts, so readings around 0.0 are right in the middle of the range and not an issue.

 

4 comments

Submitted by Abhijay (not verified) on Wed, 11/28/2012 - 06:28

I want to understand, why a short to ground is not a good practice to follow. I was experiencing a ac component when I measured voltage between LabJack ground and earth with the mains.

Submitted by LabJack Support (not verified) on Wed, 11/28/2012 - 08:31

The text in this section says "... a short to GND is usually not a good test for noise and accuracy."  The simple answer as to why, is that because on some U3s the minimum voltage on a low-voltage analog input is greater than 0.0, thus 0.0 might not be in the valid measurement range.

In terms of AC, say you connected a 100 Hz signal that varies from -100mV to +100mV (peak-to-peak).  If the minimum voltage of a low-voltage analog input on your particular U3 is +10mV, you would see the signal vary from +10mV to +100mV.  The time that the signal spends from -100mV to +10mV would simply look like a flat line at +10mV.

What you describe sounds different.  Are you saying you did some sort of measurement (oscilloscope?) of U3 GND versus some earth ground and saw an AC signal?  It is certainly normal to see small voltages at different physical locations of even the same ground.

Submitted by Rusty (not verified) on Thu, 04/11/2013 - 02:09

I am using a u3-HV for a 0-10v analog input signal on AIN3 and am getting a reading of around 1.3xxV from the labjack when the input is actually at 0V. Am I doing something wrong? Thanks

Submitted by LabJack Support (not verified) on Thu, 04/11/2013 - 08:30

The high-voltage analog inputs on the U3-HV sit near 1.4V when floating (Section 2.6.3.8).  If your signal is trying to drive to 0.0, but only pulling the analog input down to 1.3V, that suggests that your signal is very weak (high source impedance).  For comparison, jumper the analog input to GND.  For more detail I suggest you start a topic on our forum and provide a link with detail about your signal/sensor.

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