This paper evaluates the impact of external resistors used in front of resistive analog-to-digital converters (ADCs). It provides a theoretical explanation of the gain error expected, as a function of the resistor sizes, and different ways to minimize these errors. We also examine resistor tolerance and the ADC’s input impedance effects of the different calibration options. Beyond the theoretical study, bench measurements compare several devices to prove the excellent accuracy reached with on-chip gain calibration features.