Why does the graph's amplitude increase with time when using multiple channels?

[engineer 202]

I am trying to use the attached file to acquire data from three channels and to continuously plot the information on a waveform chart. However, when the data is being acquired using a string to build an array, as the information is plotted it goes up in value indefinitely when the actual voltages values are not above 2 volts each.

Can anyone give me some insight? Thanks.

[RIO-Todd]

Have you tested these channels in Measurment and Automation Explorer (MAX) to see if the voltages are doing the same thing there? If you open MAX and right-click on your DAQ device within devices and interfaces and select test panel, you will be able to test analog input channels. I would suggest to try this and see if you see the same increase in voltages.

What type of signal are you acquiring? Is it ground referenced or floating? If you are measuring a floating signal and are taking differential measurements, you should add bias resistors. I have seen the same thing happen without bias resistors when taking differential measurements. Adding bias resistor gives the amplifier on the board a path to discharge to ground. The E Series Manual shows how to con
nect bias resistors on page 4-15 and is linked below. I hope this helps.

E Series User Manual
http://www.ni.com/pdf/manuals/370509b.pdf

Regards,

Todd D.
Applications Engineer
National Instruments

[engineer 202]

Thanks. This solved the problem.