DC measurement errors due to auto-zeroed amplifier noise

An empirical fit to the noise voltage source spectrum of an auto-zeroed operational amplifier is used to calculate the variance in the measurement of the amplifier's dc input voltage or its input offset voltage. Convenient closed-form solutions for the variance and hence the normalized standard error are obtained. This allows the selection of the integration time required to achieve a desired accuracy in the measurement of the dc voltage if the low-frequency noise plateau is known or to measure the low-frequency noise plateau using a simple circuit and dc voltage measurements.