A High Input Impedance Single-Ended Input to Balanced Differential Output Amplifier

Abstract

A high input impedance circuit to convert a singleended (SE) voltage to its differential counterpart is proposed. It allows setting the common-mode (CM) output voltage to a reference voltage vR while providing gain and a balanced output. The circuit, intended to work as sensor front-end for instrumentation applications, can be implemented by using available commercial devices, thus proving a solution for the board-level design of instrumentation systems. It is based on operational amplifiers and can be tailored to specific requirements such as low noise levels, low bias currents, or limited power supply voltages. The CM output voltage is controlled by a closed-loop scheme that results in a very good balance between circuit outputs. In order to test the proposed topology, two single-ended to differential output amplifiers were built and tested: a low-noise amplifier that presents a noise of 70 nVRMS in a 0.1 Hz-1 kHz bandwidth with a noise spectral density lower than 2 nV/ √(Hz) for higher frequencies, and a low input bias current amplifier for coaxial piezoelectric sensors.