An ultra-low noise fully-differential amplifier

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 <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="eₙ" display="inline"><m:mrow><m:mi>e</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> voltage noise spectral densities as low as 1 nV √Hz ⁄ but present high <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="eₙ" display="inline"><m:mrow><m:mi>e</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> current noise spectral densities <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="iₙ" display="inline"><m:mrow><m:mi>i</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> of a few pA/√Hz. On the other hand, there are also devices with <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="iₙ" display="inline"><m:mrow><m:mi>i</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> as low as a few fA/√Hz but presenting <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="eₙ" display="inline"><m:mrow><m:mi>e</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> around 10 nV/√Hz. To obtain low values of both <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="eₙ" display="inline"><m:mrow><m:mi>e</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="iₙ" display="inline"><m:mrow><m:mi>i</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> , 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 <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="eₙ" display="inline"><m:mrow><m:mi>e</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> <1 nV/√Hz @1 kHz and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" alttext="iₙ" display="inline"><m:mrow><m:mi>i</m:mi><m:msub><m:mi/><m:mi>n</m:mi></m:msub></m:mrow></m:math> <10 fA/√Hz @1 kHz. The proposed topology finds applications such as front ends for measuring and testing instruments, industrial instrumentation, and audio circuits.