An ultra-low noise fully-differential amplifier

Abstract

A general-purpose instrumentation amplifier must be dc-coupled and has a differential input to handle both differential and single-ended input signals. It must also exhibit low input noise in both voltage and current to accommodate a wide range of signal source impedances. Additionally, having a differential output is desirable to allow direct connection to current high-resolution analog-to-digital converters (ADCs), which have differential inputs. There are commercially available devices with en voltage noise spectral densities as low as 1 nV/Hz−−−√ but present high current noise spectral densities in of a few pA/Hz−−−√ . On the other hand, there are also devices with in as low as a few fA/Hz−−−√ but presenting en around 10 nV/Hz−−−√ . To obtain low values of both en and in , a fully differential circuit topology combining discrete junction field transistors (JFETs) and operational amplifiers (OAs) is proposed. Design equations, stability analysis, and experimental results are presented. As an example, a fully differential instrumentation amplifier has been designed, built, and tested showing en<1 nV/Hz−−−√ at 1 kHz and in<10 fA/Hz−−−√ at 1 kHz. The proposed topology finds applications, such as front ends for measuring and testing instruments, industrial instrumentation, and audio circuits.