disclosed is an analog to digital conversion system that converts high speed analog signals to digital values without using a hold circuit . the system has a narrow aperture sampling circuit that samples an input signal and feeds the sampled signal to a capacitor . a resistor in parallel with the capacitor discharges the capacitor at a predetermined rate , thus the capacitor does not hold the sampled signal . the discharging signal output of the capacitor - resistor circuit is amplified and then filtered using a low - pass filter , which has essentially linear phase shift and no undershoot . the filtered signal is then amplified and fed to a flash analog to digital converter which converts the signal into a digital value . the low - pass filter creates a gaussian output and the converter is timed to convert the signal at the peak of the filter output .