Methods to reduce EMI of Delta-Sigma DACs

Control loops and sensor interfaces are one of the main applications of mixed-signal design. While the processing is done in the digital domain, analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) are needed as an interface element between the analog physical world and the abstract digital world. The DAC should ideally generate a smooth analog signal, however in reality this is not the case.

Without filtering all standard DACs will produce output frequencies that are beyond the desired frequency range. This is in particular true for Delta-Sigma based converters, a class of DACs that often finds its use in high-resolution applications where electromagnetic interferences are of particular interest since other circuit blocks of the signal processing chain might be affected by such sources of emissions.

Delta-Sigma DACs achieve their high resolution by a combination of noise-shaping and oversampling. In short, the quantization noise is processed by a feedback system in way that it is shaped and moved out of the signal band to higher frequencies. Since Delta-Sigma DACs are based on low-resolution quantizers the so called out-of-band noise is quite significant and has to be attenuated properly. As a consequence, the design effort shifts from the DAC design to the design of the output filter.

It is therefore desired, to reduce the out-of-band noise without degrading the performance of the DAC. Reducing the out-of-band noise limits unwanted emissions and relaxes the requirements on the following stages. Possible methods are shown and a solution that was implemented on silicon will be presented.