Electrical – Why are piezoelectric sensors called high impedance


Piezoelectric mics and accelerometers are called high impedance aka high Z sensors. And I know that the high impedance sensor output needs to be converted into a low impedance signal first. I think this impedance transformation is needed for carrying the signal with minimum power loss.

But my question is more fundamental here. Let’s take a piezoelectric accelerometer/mic/sensor and model it. As far as I know it is modeled as a voltage source with a series capacitor. Something like:

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Now if my model is correct for a piezoelectric sensor, why are these called high impedance sensors? Obviously Xc is frequency dependent which means it is hard to say whether the impedance is high or low here. Where am I wrong?

Best Answer

High impedance generally means that as a source, the source impedance is high. And by high, it can also mean that the signal from the sensor needs to be amplified and the amplifier needs to have a high input impedance. If the load impedance on the sensor is low, it will pull the sensor voltage lower and create error in the sensor measurement.