Measurement and Reduction of ISI in High-Dynamic-Range 1-bit Signal Generation

Abstract

This paper studies spurious signals produced by the nonlinear interaction of the previous output symbols of a digital-to-analog converter (DAC) with its current symbol. This effect, called nonlinear intersymbol interference (ISI), significantly degrades the spurious-free dynamic range of most high-speed DACs. Many papers have been devoted to suppressing level inaccuracies in multibit DACs. However, even when all levels are accurate, nonlinear ISI causes significant spurious output. This paper presents a simple and very general model for nonlinear ISI and uses it to design binary signals that can both measure and suppress the spurious tones that arise in a single-bit DAC. While the analysis in this paper is based on a 1-bit DAC, extension to multibit DACs is possible, since a multibit DAC is merely a collection of 1-bit DACs and exhibits the same nonlinear effects. Experimental verification is presented for three different hardware setups. Measurements first establish the presence of the spurious tones in the hardware, as predicted by the model, and then show how the spur level can be reduced by as much as 22 dB.