apparatus and method for calibrating an adaptive ranging a / d converter wherein a noise having known statistical properties is deliberately superposed on the reference dc signals used to calibrate the gain - ranging amplifier . the amplitude of the noise is chosen to be greater than at least one , and preferably several , least significant digits of the a / d converter at the lowest anticipated gain , yet preferably less than half the counting range of the converter at the highest gain . the reference dc signal levels are chosen to be spaced from the minimum and maximum count of the a / d at the lowest and highest anticipated gains by better than half the noise amplitude . a time average of a series of measurements of the noisy reference signals as amplified and digitized by the system are obtained . the statistical properties of the noise generator permits the system output corresponding to the reference signals to be determined to a greater accuracy than the resolution of the a / d , thereby allowing the gain and offset of the gain - ranging amplifier to be determined with greater precision than is possible by injecting noiseless dc signals alone .