An Analysis Methodology to Identify Dominant Noise Sources in D/A and A/D Converters

This paper presents a methodology of noise analysis to identify dominant noise sources in D/A and A/D converters that is applicable to any converter topology and integrated circuit technology (e.g., bipolar, MOS, GaAs) provided that the noise contribution from individual converter elements is known. The methodology is demonstrated using a simple voltage - referenced D/A topology and a Dash A/D converter. The thermal noises in an R-2R network and in a binary-weighted network are compared first as part of a theoretical analysis of the D/A converter. Then the output noise from an R-2R network with three different operational amplifier (op amp) configurations is compared to the output noise of a binary-weighted network with an op amp in an inverting summer configuration. The white and 1/f noise contributions from the op amp are included in this part of the analysis. For the flash A/D converter, the noise from a voltage reference, a resistive divider, and comparators are examined in terms of equivalent noise sources placed at the inputs of the comparators. SPICE input files constitute a noise model of a particular converter topology whose noise parameters may be changed to match actual converter noise behavior more closely. SPICE simulations are used to verify the analytical results and to examine typical noise levels in all of the circuits. The SPICE noise models allow easy identification of dominant noise sources within a given topology for a given set of noise parameters. The effects of changing the parameters of the dominant noise sources (e.g., using components designed for low noise) are examined to address design implications of the noise analysis. Finally, the effects or all noise sources on an analog-to-digital-to-analog (A-D-A) converter system are examined.