I have a doubt of a charge amplifier with a piezoelectric sensor

Hello all,

I have a question about the charge amplifier with a piezoelectric sensor as shown above.

The first image shows a piezoelectric sensor and its equivalent circuit and the second one shows the charge amplifier with the sensor.

Since piezoelectric sensors have a very high impedance, Rp, the amplifier should have a very high input impedance in a voltage mode. Otherwise, the amplifier should have a very low input impedance in a charge mode to feed all the charge from the sensor to the feedback capacitor.

However, if I put a resistor, Ri, as shown above, is it fine?

I think the input signal to the amplifier will be dropped due to the Ri, but a TI application note (SLOA033A) shows the circuit.

I wonder how the circuit works with a piezoelectric sensor having very high impedance.

Ri will affect the high frequency breakpoint and gain limit

Yes, that's true, but there is Rp that is very high impedance of the piezoelectric sensor. If I add Ri, it may be problematic due to the Rp. Is it correct?