NI-9250, GRAS microphone - what are the signals?

[AdamTrojak]

Hi

I am working on student's topic with theoretical sound measurements. 

There's proposition to use NI-9250 along with G.R.A.S 46BE microphone.

However I don't really understand what ranges of signals will be transferred between NI-9250 and microphone. 

The manual for NI-9250 stands, that Input voltage range is +-5Vpk, and that IEPE compliance voltage is 19V max.

Footprint stands that "(Vcommon-mode + Vbias ± Vfull-scale) must be 0 V to 19 V" 

However the documentation for microphone doesn't describes Vfull-scale, the DC bias voltage is 12V.

Can somebody connect these two specs and explain to me how it comes with Constant Current Power mode that it suppose to use?

https://www.gras.dk/products/measurement-microphone-sets/constant-current-power-ccp/product/143-46be

http://www.ni.com/pdf/manuals/378009b_02.pdf

Thank you!

[Henrik_Volkers]

Some basics to IEPE (or ICP, deltatron, ccld, ....) can be found at wikipedia and even on the ni side (somewhere... :-/)

https://en.wikipedia.org/wiki/Integrated_Electronics_Piezo-Electric

The 9250 will supply a constant current at his input .. depending on the output impedance of the IEPE device the 9250 now reads the resulting voltage (in the specified range) 

Your IEPE device is the GRAS mic and it will adjust his output impedance such that the voltage settle to 12V (assuming a 4mA current the output impedance will be 3k Ohm) .  4mA at 12 V results in a power ... part of that power is used to supply the internal electronics of the mic.

So the voltage at the 9250 input is 12V (the bias) +- signal . In the 9250 the biasvoltage is removed (by using a capacitor, that why you get a high pass filter) and the signal is digitized in a range of +- 5 volt.

[AdamTrojak]

Hi Henrik_Volkers!

Thanks for reply.

I read the wiki page, but I was confused when reading the devices specs.

So the NI9250 will provide this 12 Bias voltage and 2mA current, right? 

Then the signal voltage from the mic should be within +-5V, right?

Can you tell me what is the Vcommon-mode described in NI-9250 specs?

Thanks

[Henrik_Volkers]

No No No

Seems that you need to learn some  basics in physics (or EE) first ...

Start with Ohms law ..

Spoiler

Answer from a med student in the phy-lab: Which one, there are three ?

[AdamTrojak]

I understand it might be trivial to you, but I got confused and that's the reason I am looking for some help here. No need to be rude.

[santo_13]

Please check if this reference helps you.
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P8uwSAC&l=en-US

In simple terms, the Instrument provides a constant current, the sensor alters its resistance(not exactly but simpler terms) as per the quantity it is trying to measure to change the voltage drop across the sensor which is, in turn, picked up by the instrument as a signal. The instrument can force a constant current by adjusting its internal voltage source up to a limit called compliance limit.