Offset voltage on PCI-6123

[jchrisj]

I am using the M31 in differential mode, but I found a document on the LinearX web site that talks about how to build cables to connect these microphones.  On this page it recommends that you run the microphone on a 9V DC power supply and it says:

"The microphone output is single ended, typically 150 Ohm, and may contain DC bias.
Signal amplitudes from the microphone can exceed 10V peak."

My experience is that it does contain DC bias - about 5V in my system if I read it right. I've seen the DAC channel clipping the signal in some cases so I've installed a simple  voltage divider at the input of the BNC-2110 .

I had been using a 100K resistor to ground (or more accurately the shield ont he BNC cable) followed by a small 0.01 capacitor to try to block the DC bias. I'm now using a divider after the capacitor to try to drop the signal by about 20% in the hopes of keeping it in range.

When I tried this arrangement I saw a lot lower voltage than I expected so I've disconnected the initial 100K resistore, but I haven't had a chance to test it yet.

I haven't thought about the dBspl conversion so far. 

[sherwinlalap]

Chris,

Thanks for the info. I am also new to microphone, I knew a lot of sensor but this is my first time to use microphone from my applications. Do u know where can  buy a second hand daq? I have a lot of experiment to do, but I liketo have my own one since brand new is expensive for me now

Thanks

Fox

Philippines 

[jchrisj]

I don't know about sources for secondhand DAQs - e-bay, maybe?

I was a little surprised about the M31 microphones. I was used to Mics that generate low (millivolt range) signal levels so I was expecting to need a preamp. I even designed and built one before I got these Mics and it turns out that I don't need an amp, I need a divider!

I haven't had the chance to test the voltage levels with my new divider circuit. I'll try it and report my results.

Chris

[shaker]

Hello jchrisj,

Did You found the solution how to get rid witch DC Bias? I have the same problem now.. I bought M31 mic and i'm trying to acquire signal through DAQ USB 9006. Levels are between 3,9 up to 7V. now i want to measure dBspl knowing the sensivity of mic. if You will have any advice for me, how to measure it, I would be very greatful.

Cheers