NI 9233 and electret microphone

[cpalka]

Dear All

Recently I buy this electrect microphone with amplifier

http://www.ebay.com/itm/381845584916?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I work with it in ni usb 6009 because the amplifier board has a DC output.

The manufactur saise

"If the audio equipment you're using requires AC coupled audio, place a 100uF capacitor between the output pin and the input of your device. If you're connecting to an audio amplifier that has differential inputs or includes decoupling capacitors, the 100uF cap is not required."

I think the Ni 9233 uses AC.

How can I use a electret micro with NI9233, can I use this micro with  amplifier max4466, conect it directly to the bnc port from ni 9233 and place a 100uF between + and -???

Or is not so simple??

I have another micro with amplifier MAX9812L, this is better? But it has DC out too.

Thanks in advance

cpalka

[LocalDSP]

    The NI 9233 always generates a 2 mA DC current out of the BNC connector (IEPE supply) and therefore it's not the best product for you to use for your application. A better module would be the NI 9234 or NI 9251, do you happen to have one of these modules?

    If not you can try to get your microphone board to work with the NI 9233. However I want to emphasize that there is a small risk that the following procedure will damage your amplifier, so only try the following suggestion if you don't have other options and are willing to take the risk.

1 . Add a 5-7 kohm resistor directly in parallel with your BNC connector (between center pin and housing). This will force the DC level at the BNC input to approx. 10-14 V, a good bias value for that module

2 . Add a 10 uF electrolytic capacitor (rated for minimum 16 V, recommended 25 V !) in series between your amplifier output and your BNC input (center pin). And off course connect your amplifier signal ground to the NI 9233 BNC ground (housing). Make sure the + end of your capacitor is connected to the BNC side and the - end to your amplifier output. Note that the capacitor value is reduced from 100 uF to 10 uF due to the relative high input impedance of your modified acquisition system (~5 kohm). You can probably even reduce your capacitor value to 1 uF without affecting your performances significantly, this will also reduce the risks of damaging your amplifier board.

3 .Make sure to power on your microphone-amplifer board before you connect it to the NI 9233

Good Luck!

[cpalka]

Dear LocalDSP

It cannot be more explained, many thanks!

I will try your suggestion with time and then I will give here the feedback.

I explain why the ni9233, we have a very expensive equipement to measure sound and the licences to exepensive.

The problem is that we have a prototype in our lab commanded by labview and other NI boards. To join now the sound (noise) with Bruel equipement I dont know how to do it. So I have an unsed ni 9233 in the lab and a sound Enginier saise me to start learning with cheap microphones, electret. With NI6009 and soud and vibration module it works (I can see data, but need to learn understanding data). In the future will be an expensive microphones that connects directly to the bnc port of the 9233.

Thanks

CPalka