Ultralow-noise programmable voltage source

To avoid introducing additional noise sources while making low-frequency noise measurements, batteries are normally used instead of electronic power supplies. This paper presents an alternative solution by describing the design, construction, and testing of an ultralow-noise voltage source. Such a power supply can be computer controlled and has a typical noise level two orders of magnitude below that of similar commercial instruments. Some typical values of the spectral density of the voltage fluctuations at its output are: (10/sup -12/, 10/sup -15/, 10/sup -16/) V/sup 2//Hz at (0.01, 0.1, and 1) Hz, respectively. These noise performances are almost independent of the supplied current, with a degradation of less than 3 dB up to 400 mA. A special algorithm for digital-to-analog conversion, using passive devices with 1% tolerance, ensures a resolution of 2.5 mV and an accuracy better than /spl plusmn/1.5 mV over the entire output range from 0 to 8 V.