PCI-6251: analogue readings suddenly drop resulting in necessary reboot

[avatar1]

I am using a PCI-6251 card in a testsystem where small loudspeakers are tested. Next to the loudspeakers there is a microphone and a pre-amplifier which is connected to an analog input on the DAQ-card. The test system runs Teststand and Labwindows CVI which runs a sequence to test the power of the speaker at different frequencies.

The problem is that I can only run this test 5-10 times and then suddenly there is a negativ offset which persists when I continue to test. The only way to solve this that I have found is to reboot the computer, which I suppose resets the DAQ-card. After the reboot it works fine again for a few tests and then it begins to fail again.

Does anyone know what I can do to solve this issue? Is there something wrong with the DAQ-card?

Thank you,

Jimmy

Message Edited by avatar1 on 03-19-2008 05:36 AM

[preston_johnson]

You might consider an AC coupled DAQ card which will remove the DC offset with an AC coupling filter.  A cost effective one is the USB-9233, http://sine.ni.com/nips/cds/view/p/lang/en/nid/202100

The pre-amp and the microphone typically are used with Dynamic Signal Data Acquisition devices, and the maker of the microphone is less interested in the DC drift, than the AC accuracy.  The High impedance of the DAQ board may also be a limiting factor. 

Whose microphone are you using?  Are there any particular software analysis of the data you are expecting to do?

[avatar1]

Tank you for your response but the thing is that we already have this card installed and I do not have direct access to the source code.

I dont know which microphone it is, could be from Shure. It is connected to a Shure rpm626 pre-amp and a Phantom power supply.

The problem is that it is measuring perfectly for a while and then suddenly it gets this offset which seems to be about the same amount all the time. When this offset has occured all following measurements will also have this offset. But when I restart the computer it will return to showing correct values again, for a little while at least...

[preston_johnson]

You will need the source code of the application so that you can insert a device reset - a software function  - into the program.

You can also build an ac-coupling circuit using a resistor and a capacitor to remove the DC drift of the signal.  Consider consulting with your electrical engineering team.