Redesign of the AD820 Single-Channel Circuit for the Development of the aRD820 Low-Noise Rail-to-Rail Operational Amplifier

TL;DR

This paper details the redesign and development of a low-noise, low-power rail-to-rail operational amplifier, aRD820, adapting an existing AD820 design to specific technological capabilities and verifying performance through simulations and measurements.

Contribution

The work presents a modified circuit design of the AD820 for improved performance and manufacturability, including detailed circuit modifications and validation through simulations and physical measurements.

Findings

The aRD820 meets targeted low-noise and low-bias specifications.

Simulation results closely match the original AD820 characteristics.

Physical measurements confirmed the successful realization of the design.

Abstract

The task of this work was to design and later produce a low-power (single supply 5 - 30 V, dual supply +-2.5 V and +-15 V) rail-to-rail operational amplifier aRD820 with low voltage noise (<4 uV, p-p. 0.1 to 10 Hz), ultralow input bias current (< 15 pA), and low offset voltage (< 500 uV) characteristics. Similar characteristics are presented by Analog Devices chip AD820. Thus, the task of the design team was to adapt the prototype circuity of AD820 to our technological capabilities, modify the circuit, if necessary, to eliminate any deficiency of the prototype. The input stage module got source followers at the input of the operational amplifier. Second stage module was modified to be more symmetric. The output stage module obtained additional resistors and capacitors to achieve a frequency compensation. One FET transistor in the current reference module was substituted by other elements. Simplified electric schemes of these modules of AR820 and aRD820 are presented. The performance of modified electric schemes of modules was tested in Simulink software. Simulations of the full electric scheme for aRD820 were made and showed that it demonstrates similar characteristics as AD820 data tables. Later production of the aRD820 chip and measurements demonstrated that the planned characteristics of the operational amplifier were met.