DC offset errors believed due to hardware when running MAX

[josh100482]

Hi,
I am having trouble testing two microphones against each other with the S&V toolkit.  I would like to test my microphone against another.  However my microphone has a DC offset of -5 Volts.  This DC voltage is leaking over to the reference microphone (especially at higher sampling frequencies).  I do not understand how or why this is possible.  (I am using a PCI 6251 DAQ card and a BNC 2110 for hardware).  If anyone could shed some light on this that would help a great deal.
Thanks,
Josh

[Chris_K_]

Hi Josh

The problem you are describing sounds like it is related to ghosting, as you mention that the problem gets worse as you increase the sampling rate.  The attached document explains ghosting, and steps you can take to avoid it. 

How Do I Eliminate Ghosting From My Measurements?

http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF?OpenDocument

I would suggest reading in three channels, ai0, ai1, and ai2 in that order.  Attach your microphones to channels ai0 and ai2.  This will give the MUX/ADC some more settling time (the time the task would be taking a measurement on ai1) and eliminate the voltage leaking from one channel to the next.  Ensure that you set the scan list in this order so that there is extra time between the readings. 

If you are still running into problems, here are a few things that I need to know.  Are the microphones you are using powered? If so, what excitation do they need, how are you providing this excitation, and how are you connecting the microphones to the BNC 2110?

Regards

Chris_K_