A simple method to measure and improve linearity of flash ADCs used in integrated VME ADC modules

To measure and improve linearities of the Flash ADCs used in the integrated VME ADC modules, pulses with exponentially falling tail were digitized and the counts of the resultant digital codes were recorded. The obtained histogram of the codes was fit to a function to deduce a smoothly varying curve which corresponds to that obtained in the case of ideal linearity, and was compared to the fit to calculate width as well as differential nonlinearity of each code. The obtained differential nonlinearities go down to as low as ~0.7 LSB. A simple method is proposed to improve the nonlinearity using the differential nonlinearities in the post-processor following the ADC.