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

[CHL]

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.

 

[D.A.(Tony)Stewart]

Ri will affect the high frequency breakpoint and gain limit

 

[CHL]

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?