Cross contamination of signals on the NI USB-6211 board?

[jasonjps]

I have a setup with 7 electret condenser microphones connected to a NI USB-6211 board.  The seven 3-lead microphones have the power lead connected to both of the 2 AO ports at 1.3V (drawing just a fraction of the max amps but I connected both analog outs just incase).  The each signal lead is connected to each of AI0-6, and the ground is connected to AO GND, AI GND, and differential negatives.  I've tried this in differential mode as well as RSE and found that the signals seem to influence each other.  This became really aparent when I connected an active speaker to AI7 differential input.  When connecting and disconnecting the speaker signal, I clearly saw peak to peak values of AIO change.  In addition, I also noticed that if I didn't have a good connection on one of the analog inputs, the signal read by labview would just take on the value of the adjacent signal.  And yes, the wires for most of the length are shielded.

So I guess I have 2 questions:

1) Is there a recommended way to connect several 3 lead electret condenser microphones? ( I heard I should use differential for sub 1V signals, but it seems odd to connect the ground lead to the differential port just to turn around and connect them all together and them the the AO ground anyway)

2) Why are the signals affecting each other?  Is there a way to fix this? 

[johnsold]

What is the output impedance of the microphones?  It sounds as though you are getting coupling through the capacitances of the multiplexer.  This effect is much more common when source impedances are high.  You indication of coupling to an unused channel is strongly suggestive of the capacitive coupling issue.

Your signals are clearly not differential so a single ended connection makes more sense.

Lynn

[jasonjps]

The nominal output impedance is 4400Ohms. 

http://www.knowles.com/search/prods_pdf/FG-23629-P16.pdf

Thanks:)

[johnsold]

Look at page 2 of the NI USB-621x Specifications.  It shows errors and settling time for various source impedances.  After 5 us with Zsource < 100 ohms the error is about 15 ppm (1 LSB).  For Zsource = 5000 ohms (closest value shown to your microphone impedance) the error is ~7500 ppm.

While these values may not correspond closely to your observed results, I think it indicates that the source impedance is likely the cause.  A low output impedance buffer for each microphone is probably the best fix.

Lynn

[jasonjps]

How do I do a low output impedance buffer?  I'm not even sure I know what that is.

[johnsold]

Find someone skilled at electronics.  While not a difficut thing to design or build it is beyond the scope of this Forum and would be a challenge for you if you do not "know what that is."

Lynn

[jasonjps]

Is this just a voltage follower?  I've done voltage followers before, but for the reason of needing a high impedance on the input.

[johnsold]

Yes.  A voltage follower typically has high input impedance and low output impedance.  In this case the latter is more important.

Lynn