IEEE Instrumentation and Measurement Technology Conference Ottawa, Canada, May 19-21, 1997 zyxwvuts Ultra low noise current sources C.Ciofi, R.Giannetti, V.Dattilo, B.Neri. Dipartimento di lngegneria dell' informazione: Elettronica, Informatica, Telecomunicazioni v. Diotisalvi zyxwvu 2, 1-56126 PlSA (Italy) Phone +39 50 568538 Fax +39 50 568522 E-mail: ciofi@iet.unipi.it zyx Abstract - The current sources which are normally used in research laboratories are not suitable for low noise measurement systems because of the unacceptable level of low frequency noise which they introduce in the measurement chain. The most important source of low frequency noise in such instruments is the solid state device (usually a Zener diode) which is used as voltage reference. By using a novel circuital topology in which a lead battery which does not supply current is used as voltage reference, we have been able to design an ultra low noise current source characterized by a low-frequency noise level some orders of magnitude lower than that of similar commercial instrumentation. The design, realization and testing of such current source is presented in this paper, I. INTRODUCTION. In low frequency noise measurement systems (k10 Hz), it is often the case that the device under test need to be biased at constant current. Traditional solid state current sources can not be used to this purpose because of the high level of noise which they introduce. Sometimes a battery together with a high value series resistor can be used to simulate a current source. However, such solution becomes impracticable when the current which must be supplied is in the range of hundreds of mA or more. Moreover, the noise produced by the battery dramatically increases if the supplied current overcomes a certain value zyxwvuts I, which depends on the type and the capacity of the battery [l]. Normally I, is of the order of magnitude of the current at which one hundred hours are sufficient to completely discharge the battery. Finally, it is not possible to maintain the supplied current constant because of the discharge of the battery during the measurements. To overcome the above mentioned problems, purposely designed ultra low noise current sources must be developed. The topology and the principles of operation of a rather simple ultra low noise solid state current source are described in this paper. By means of an accurate selection of the active and passive components, it has been possible to obtain, in the very low frequency range, a noise level which is orders of magnitude lower than that of instrumentation of the same type available on the market. II. SYSTEM DESIGN It is a well known result that batteries behave as low noise voltage sources provided that the current which they supply is very low [ l ] . In other words, it is possible to employ a battery as a very low noise voltage reference. In fact, in low frequency noise measurements, a high accuracy in the value of the supplied current, as that which could be obtained by means of a Zener diode, is normally not important. It is much more important to keep the background noise of the system to a minimum. In the circuit reported in Fig.1, already proposed in [2], a battery which does not supply current is employed for biasing a low-noise FET (8 2SK146 by Toshiba in parallel) used as current source. 1 \. . .,. .. .. ,' - Fig.1. Schematic of the ultra low noise current source 0-7803-3312-8/97/$5.0001997 IEEE 1486 Authorized licensed use limited to: Rochester Institute of Technology. Downloaded on May 26,2010 at 14:48:29 UTC from IEEE Xplore. Restrictions apply.