USB-6218 Charge Buildup

[captorion214]

I've just started using an NI USB-6218 DAQ to take analog differential measurements (from electric field probes). Currently, I have the device hooked up to my computer and have each differential channel shorted with a large 1M ohm or larger resistor.

It appears that, while the DAQ is plugged in, but not being used, voltage is building up on each channel of the DAQ. When I run my VI (still in development), I can see a relatively large initial voltage spike when I first start reading the pins. After this initial spike, each channel performs as expected and gives voltage readings at the noise floor of the device. The size of the initial voltage spike appears to depend on how long the DAQ is plugged in but not used. For example, if I don't use the DAQ for 5 minutes, the initial voltage spike is no more than +/- 0.2V. Not using the DAQ for 20 minutes produces an initial spike of up to +/-0.8V. The exact value of the initial voltage spike varies from channel to channel.

I have tried adding a resistor between the negative side of the differential channel to "AI GND" and from the positive side to "AI GND" but this does nothing to the initial spike. 

Other misc. information:

The DAQ device is brand new, I'm the first person to use it. I'm running labview 8.5 and reading all 16 differential channels. The range on all channels is set to +/- 10V. Sampling is done 2 samples at a time at a rate of 100 Hz. 

Thanks in advance for any help or advice you can give!

[Hillman]

Hi Captorion,

Good afternoon and I hope your well.

So alot of your setup configuration depends on what type of source your trying to acquire from. The table belows tries to explain, 

 

Please take a look at this link for more info,

Field Wiring and Noise Considerations for Analog Signals

http://zone.ni.com/devzone/cda/tut/p/id/3344

I think the voltage you are seeing is because the 1M is goign to build up a voltage as the device will have a leakage current. It is suggested to use kOhm resistors. Why are you shorting out the inputs ? 

If you could maybe explain what your trying to acheive then maybe I can advise further?

Hope this gets you started.