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AIN0-3 Labjack U3-HV

output voltage of 0.6 V

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#1 mfebbo

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Posted 23 October 2013 - 06:04 AM

I measure the voltaje with a mutlimeter of AIN0-3 with respect to ground and find an output voltage of 0.6 Volts. Should I not measure 0Volts with respecto to ground? 

#2 LabJack Support

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Posted 23 October 2013 - 08:32 AM

0 volts means you are driving 0 volts.


No connection is not 0 volts ... rather no connection means that the voltage is not defined, so the value is generally not easy to predict.


See the following:




With the specific case of the high-voltage inputs on the U3-HV, they do sit at a predictable voltage of 1.4 volts when floating.  When you connect your DMM, it likely has low enough impedance that it causes that weak voltage to change to 0.6 volts.  Do you see that if you watch the AIN0 reading in the test panel in LJControlPanel?  That is, if AIN0 is floating the test panel reports about 1.4 volts, but when you connect your DMM the test panel and the DMM both report about 0.6 volts.


When you connect a driven signal with low enough source impedance, that signal will drive the analog input to the correct voltage.  For example, if you connect VS or GND you will see ~5 volts and 0 volts.  Or you can connect DAC0 and see that AIN0 will read whatever you set on DAC0.


Appendix A specifies that the source impedance of your signal should be less than 10 kohms (or 200 kohms if using LongSettling).



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