Optimization of the input impedance of a low-noise unipolar powered amplifier

Recording systems in neurophysiological research have to deal with high sensor impedances of 1 M Omega and above, while having pretty low signal amplitudes of several mu V. In cooperation with the German Primate Center Gottingen the Institute of Electromechanical Design develops a wireless recording system which can be carried by animals. Since common circuits and electrical parts for noise reduction used in stationary on-the-shelf systems are too space demanding for our device, we designed a simple input stage with an instrumental amplifier followed by cascade of programmable gain amplifiers. The widespread approach of dimensioning the amplifier's input resistance as high as possible has appreciable disadvantages in design of compact mobile recording systems, such as leakage currents and parasitic resistances. Furthermore thermal noise increases significantly for broadband recordings. We could show, that accepting lower input impedance leads to an almost optimal signal to noise ratio and reduces design troubles for mobile devices. (C) 2011 Published by Elsevier Ltd.