# Why would my signal from a piezoelectric device changes while using a voltmeter and electrometer?

When we apply a force to a piezoelectric material it produces some potential difference.

I was measuring with Keithley nanovoltmeter and Keithley electrometer 6514.

I am getting different output in both the devices when applying the same force to the piezo.

Please let me know which one is correct and why?? the change is not small and its from 10 to 100 times

• What is the difference in Zin?? – Tony Stewart EE75 Sep 19 '17 at 16:29

nano voltmeter specs >10 GΩ

eleectrometer specs > 200 TΩ

If result changes 100x then DUT Z is > 100x10G = 1 TΩ

If result is dynamic and changes 10x then consider fixture capacitance loading. One needs a Charge Amplifier for this or special 0 pF fixture right at instrument.

• Charge is proportional to pressure
• Rate of charge is equal to current Ic=dQ/dt = C dV/dt
• thus dynamic pressure from acceleration (ac) is proportional to Iac into load R. DC Leakage resistance must be > TΩ to measure static pressure which makes this difficult to do.
• this is the fundamental property that occurs in accelerometers useful for sensing g and integrating to v but not useful for static pressure. For this they use load cells.
• piezo effect also makes ceramic caps microphonic.
• +1 but you forgot to mention the why.... Piezoelectric materials have a really high output impedance, so Zin matters a lot. – Trevor_G Sep 19 '17 at 16:41
• @P.Rajagopalan Current available from Piezo elements is tiny, while voltage is large. You might consider it more of a current source, at least for static forces. For dynamic force, you likely have to consider a model including resonance and parallel capacitance. – glen_geek Sep 19 '17 at 16:59