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

[davey31415]

Hi, 

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?

Thanks, 

Dave

[Jordan F]

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?