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USB 6211 analog input switches between input value and zero volts only when DMM is connected



I'm using a USB 6211 to continoulsy acquire analog input voltage in differential mode (LV 8.5).  If I attach a DMM to a terminal from the acquired signal, the signal shown on a waveform chart begins to change between the actual signal value and zero volts.  Both DMM leads do not need to be attached- the negative alone seems to do this. 


I'm using a Fluke 179 and have verified that the DMM is set to DC voltage input and the leads are connected to the proper  DMM terminals. 


Any ideas on what is happening?





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Hi Dave,

What is the signal you are measuring and how do you have it connected?  Are you able to get the correct reading with the USB-6211 if you do not have the DMM connected?  Can you post a screenshot of what behavior you are seeing when you are measuring with the USB-6211 when the DMM is connected vs. not connected?   

Jordan F
National Instruments
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