Supercapacitors in bias systems for low frequency noise measurements

We explore the possibility of employing supercapacitors in the design of very low noise programmable voltage and current sources to be used for biasing the device under test in low frequency noise measurements. This is accomplished by taking advantage of the large capacitance of supercapacitors for realizing a low pass filter with a very low frequency corner for filtering out the noise produced by a solid state high accuracy DA converter. In order to obtain the desired attenuation of the noise down to a few hundred mHz, a very low frequency corner must be employed, thus resulting in very long transients (in the order of a few hours) upon each voltage change. This problem is specifically addressed in this paper. Indeed, we have designed a programmable voltage reference capable of generating voltages in the range 0–2.5 V with a resolution of about 2.5 mV, an accuracy better than 2 mV and a settling time of about 200 s. The output noise is below 4×10−16, 5×10−18, and 1×10−18 V2/Hz at 100 mHz, 1 Hz, and 10 Hz, respectively. The application of such a low voltage reference for the realization of a very low noise programmable current source is also discussed and demonstrated.