A method for sensing includes connecting an input of a trans-impedance amplifier (TIA) to a first terminal of a sensor, which generates a current in response to an input signal that is incident on the sensor, the signal comprising pulses of a characteristic duration. A resistor is connected in series between a second terminal of the sensor and a power supply, which is set to drive the sensor at a selected voltage. A capacitor is connected to the second terminal in parallel with the sensor and the TIA and in series with the resistor. An upper limit is set on the current that is to be input to the TIA from the sensor, and respective values of the resistor and the capacitor are chosen, responsively to the characteristic duration of the pulses and the selected voltage, to prevent the current input to the TIA from exceeding the upper limit.