Drift and low frequency noise

To compare errors arising in a measuring instrument from drift and low frequency (1/f) noise, the behaviour of the latter in the time domain is calculated. The standard deviation is expressed in terms of the cut-off frequency and integration time of the instrument, and the frequency index of the noise, which can be measured at audio frequencies. Optimum values of these quantities are discussed. The theory is illustrated by reference to measurements on m.o.s. transistors which indicate that current noise may sometimes be the main cause of `drift¿ for periods between a day and a year.