Problem reading voltage of my piezoelectric sensor on NI 9205

[kdj2000@postech.ac.kr]

Hi 

I'm trying to measure my piezoelectric sensor on NI 9205(which is connected to cDAQ 9178) via DAQExpress.

I found out that all the signals that I generated from function generator can be read on NI 9205 pretty accurately. 

But when I try to measure my piezoelectric sensor, the voltage is measured much larger than its value and it doesn't come down to 0V instantly (while it should be) but slowly comes down to Value.  Like there is a capacitor attached. 

I tried every mode (differntial, RSE, NRSE) but it shows the same result.

But when the piezoelectric sensor is connected to both Oscilloscope and DAQ, the signal is measured correctly.

Maybe the input impedence of NI 9205 is too low for my sensor? or is it about floating issue?

Is there way I can controll this situation so that I can only use NI 9205 for the measurement?

I also attached signal pictures below. 

Thank you

  

[Henrik_Volkers]

your piezoelectric sensor is your capacitor 😉

the drift you see is the bias current of your 9205.

quick help: add a 1 Meg Ohm resistor over your input. But be aware that if you use more than one channel of your multiplexed input DAQ, you will see settling time errors.

Your piezo create a charge due to a force on it. together with the capacity of the sensor AND the cable (and the input capacity of the 9205) this charge results in a voltage on that capacitor. The additional resistor unloads the capacitor .. resulting in a high pass ..

best would be to use a charge amplifier (CA)  to measure the output of such sensors. depending on the CA you still have a high pass, but with a much lower frequency 😉   and you are mostly independed of cable length and capacity (well, mostly) and have a low impedance output to your 9205