An Ultralow-Noise Fully Differential Amplifier

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 e(n) voltage noise spectral densities as low as 1 nV/root Hz but present high current noise spectral densities in of a few pA/root Hz. On the other hand, there are also devices with in as low as a few fA/root Hz but presenting e(n) around 10 nV/root Hz. To obtain low values of both e(n) and i(n), 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 e(n)<1 nV/root Hz at 1 kHz and i(n)<10 fA/root Hz at 1 kHz. The proposed topology finds applications, such as front ends for measuring and testing instruments, industrial instrumentation, and audio circuits.