Piezoelectric sensors — accelerometers, force gauges, ultrasonic transducers, knock sensors — are unusual: they generate charge proportional to mechanical stress, not voltage or current. Yes, you can read them with a high-impedance voltage buffer, but the output voltage depends on the sensor's self-capacitance plus cable capacitance. Swap a 1 m cable for a 3 m cable and your sensitivity changes. Unacceptable for calibrated measurement.

The charge amplifier fixes this by clamping the sensor's terminal voltage to zero and dumping all generated charge into a known feedback capacitor.

Topology: An op-amp in inverting configuration with a capacitor C_f in the feedback path (no input resistor — the sensor connects directly to the inverting node). The inverting input is a virtual ground, so cable capacitance sees no voltage swing and contributes nothing to the transfer function.

Transfer function: V_out = –Q / C_f

That's it. Output voltage depends only on the charge Q from the sensor and your feedback cap. Cable length, sensor capacitance, connector quality — all irrelevant to the gain.

The DC drift problem: A pure capacitor in feedback provides no DC path, so bias current and offset voltage will ramp the output into the rail within seconds. You must place a large resistor R_f in parallel with C_f. This creates a high-pass corner at f_c = 1 / (2π R_f C_f). Choose it below your lowest signal of interest.

Worked example — an IEPE-style accelerometer: Sensor charge sensitivity is 10 pC/g. You want 100 mV/g at the output. Solve C_f = Q / V = 10 pC / 100 mV = 100 pF. For a 1 Hz high-pass corner: R_f = 1 / (2π × 1 × 100 pF) ≈ 1.6 GΩ. Yes, gigaohms — use a low-leakage discrete resistor (or a T-network of two smaller resistors) and a JFET or CMOS op-amp with sub-picoamp bias current (LMC6001, OPA129, ADA4530).

Practical rules of thumb:

• PCB leakage will destroy you — use a guard ring around the summing junction, driven by the op-amp output.
• Keep the input trace short and clean the board thoroughly; even flux residue conducts nanoamps.
• Feedback resistors above ~100 MΩ often use glass-encapsulated or specialty parts; regular thick-film resistors leak.
• Add a small series resistor (10–100 Ω) at the sensor input to prevent instability with long cables that add capacitance to the summing node.

Modern IEPE accelerometers hide the charge amp inside the sensor housing and output a low-impedance voltage over 2-wire constant-current bias. But when you need extreme temperature range (600 °C+ engine sensors) or custom bandwidth, external charge amps are still the answer.