A Novel Method of Digital-to-Analog Converter Combination for Precise Digital Control in Closed Loop Systems

Abstract

For precise digital control applications, high-resolution feedback is essential to achieve the required performance. Precise control systems with analog input-output and digital processing are generally limited by the resolution of digital to analog converter (DAC). DACs available for space use are limited in terms of resolution and performance. This paper presents a novel approach of combining two DACs to achieve higher resolution in closed-loop control systems. DACs can be combined with an overlapping range such that nonlinearity of higher significant DAC does not cause oscillations and instability when lower significant DAC rolls over. Flowchart of the algorithm for combining two DACs is presented, along with a detailed analysis. Simulation results are also shown elaborating the behaviour of DACs in non-overlapping and overlapping combination methods. The non-overlapping combination may result in oscillations, while the overlapping combination can settle the loop after a small settling time. An implementation example of a digital lock-in amplifier for the Rubidium atomic clock, showing the realization of a coarse-fine DAC combination to achieve 20-bits resolution with two 12-bits DACs, is also presented. This approach will result in achieving better performance in small-bandwidth digital control systems.