Disclosed is an electronic system for resetting the voltage of a charge-sensitive pre-amplifier having input from an X-ray detector and output to an ADC. The pre-amplifier gain is increased so that the RMS ADC noise is less than 1% of a representative digitized X-ray signal. The reset logic is configured to avoid loss of X-ray counts and to prevent the pre-amplifier output being outside the allowable input range of the ADC. Reset is initiated when the pre-amplifier output rises above an upper level, which is below the maximum allowable ADC input. Reset is also initiated when a pile-up event is detected, provided that such reset will not cause the pre-amplifier output to fall below the minimum allowable ADC input. At each reset a known amount of charge is removed from the pre-amplifier, and the reset time is continuously adjusted to ensure that the charge amount does not drift.