A very low noise, high stability, voltage reference for high sensitivity noise measurements

In this paper we demonstrate that by exploiting the non linear characteristic of low noise PN junction diodes, a very low noise, high stability voltage reference can be obtained starting from a conventional solid state series voltage reference. In order to obtain such a result, a series connection of N identical diodes is supplied in the forward region of the I-V characteristic by means of a proper resistance. While the DC voltage drop across the diodes can be a large fraction of the voltage supplied by the reference, the noise introduced by the reference itself is reduced by a much larger factor because of the low value of the small signal equivalent resistance of the diodes. In its simplest implementation, such a voltage source would suffer from a relatively high temperature dependence of the supplied voltages because of the intrinsic properties of PN junctions. However, by resorting to a proper temperature control circuit, high stability can be obtained. As an example, by employing an AD586 voltage reference and with N = 4, a 2.560 V reference has been obtained with a stability over temperature better that 50 mu V/degrees C and a voltage noise as low as 2 x 10(-15), 6 x 10(-17) and 1.5 x 10(-17) V-2/Hz at 100 mHz, 1 Hz and for frequencies larger than 10 Hz, respectively.