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NI 9234 input impedance is too low?


I am interested in acquiring hydrophone data with the NI 9234 using piezoelectric hydrophones.  The manufacturer requires a very high input impedance to achieve accurate low frequency response, which we are interested in.

According to the manufacturer, to achieve accurate operation at 10Hz, the input amplifier must have an input impedance over 2.5Mohms.

If I am reading the specifications correctly, the 9234 only has an input impedance of 305kohms in differential mode.  Using the hydrophone manufactuer's equation, we won't get accurate information below 86Hz, which is unacceptable.

I guess my question is.....does the 9234 really have such a low input impedance, or am I reading the specs wrong?  I thought at least 1Mohm was pretty standard for DAQ devices.

If I am correct that the impedance is really too could I go about increasing it?  A bridge made of precision resistors?

Thanks for any input!  I'm a civil engineer with moderate electrical experience, so I appreciate any info!

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Message 1 of 5

After a quick glance at the manual for the NI 9234, I would say that it is a poor choice for a hydrophone application. It looks like it was designed to be used with IEPE sensors. To use this effectively with your hydrophone you should probably use an external preamplifier with the necessary high input impedance.



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could you please explain the relationship between the input amp impedence and the output frequency of the hydrophone ? (the equation what you mentioned)



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Message 3 of 5

The sensor is piezo, so it is like a capacitor.  So you get the RC corner frequency, and since the capacitance of the sensor is a fixed value, the R sets the corner frequency. 

Message 4 of 5

I would try to read the piezo sensor with a charge amplifier topology 🙂 (or/and insert a charge to IEPE conerverter) , but I assume the sensor calibration is done in voltage mode.

OK , I'm a accelerometer guy, and if you (only)  have a hammer, every problem looks like a nail 😄
Greetings from Germany

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