Setting a Piezoelectric Charge Amplifier’s Low Frequency Response

A piezoelectric sensor can respond strongly to a changing force while failing to maintain a steady reading under a constant load. The sensor and charge amplifier form a dynamic measurement system. Review the intended event duration before trying to remove every observed decay through calibration.

Separate charge gain and discharge

In a simplified charge amplifier, transferred charge produces an output determined by the feedback capacitance. The feedback resistor provides a discharge path and affects low frequency response. The TI application note on piezoelectric sensor conditioning describes these functions and explains why the resistor prevents indefinite charge accumulation and drift toward saturation.

For an illustrative ideal calculation, 10 picocoulombs applied to a 100 picofarad feedback capacitor corresponds to 0.1 V in magnitude. Polarity depends on sensor and circuit connections. This charge gain calculation does not establish behavior at every frequency or with every cable.

Relate decay to event duration

The feedback resistance and capacitance define an RC time scale in the simple model. A 100 megohm resistor and 100 picofarad capacitor have a 10 ms product. Compare the expected decay over the event with the allowable measurement error rather than selecting resistance solely to maximize low frequency sensitivity.

A longer time constant can retain transient information longer, but it also changes recovery from disturbances and the consequences of leakage. Define how quickly the system must return to a usable baseline after a large mechanical impulse or connection event.

Review real input conditions

Include sensor capacitance, cable capacitance, amplifier input current, protection leakage, and noise. Check output range at the largest expected charge event. A circuit designed for small vibration signals may saturate during handling even if normal operation is well within range.

Provide a defined procedure for connecting the sensor and starting acquisition. Cable motion and connector handling can introduce disturbances that should not be interpreted as the intended physical stimulus. Record the cable type and length used for characterization.

Verify with time and frequency evidence

Apply a repeatable dynamic stimulus and compare peak response, decay, and recovery. Where practical, use an electrical charge injection method with a known capacitor to check the electronics separately from the mechanical sensor.

Save feedback component values, sensor configuration, event duration, and the measured response. State the useful frequency or time range explicitly. A charge amplifier that correctly measures a short impact should not be described as a general static force instrument without additional evidence supporting that application.