An ultra-low noise fully-differential amplifier

摘要: A general-purpose instrumentation amplifier must be dc-coupled and have a differential input to handle both differential and single-ended input signals. It also must 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 voltage noise spectral densities as low as 1 nV√ Hz⁄ but present high current noise spectral densities of a few pA/√ Hz. On the other hand, there are also devices with as low as a few fA/√ Hz but presenting around 10 nV/√ Hz. To obtain low values of both and, a fully differential circuit topology combining discrete Junction Field Transistors (JFET) and Operational Amplifiers (OA) 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< 1 nV/√ Hz@ 1 kHz and< 10 fA/√ Hz@ 1 kHz. The proposed topology finds applications such as front ends for measuring and testing instruments, industrial instrumentation, and audio circuits.