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Analysis of Distortions Caused by Impulses in Switching Transistors Sources in Current-Steering DAC  

 Svizev G.A.
 Ionov L.P.
 Mukhin I.I.
 Zolotarev A.V.
Date of publication

 During a switching of current-steering DAC cells (bits) voltage impulses appear in sources of switching transistors. These impulses penetrate directly into the DAC output signal, and, more importantly, they create code-dependent disturbances in common (global) biasing circuits of current cells leading to spurs in output signal spectrum.
These disturbances and spectrum distortions caused by them are widely reported in the literature [1]-[10] as one of the main reasons for deterioration of AC characteristics of current-steering DAC, as well as methods to reduce these distortions.
However, in the found publications these disturbances and distortions are considered only on a qualitative level, without mathematical models and their analysis. At the same time the authors do not specify numerically the im-pact of the mentioned disturbances on the key AC parameters of the DAC (SFDR, IMD, etc.), neither by assessment nor even on simulation results for specific schemes with particular parametric adjustment.
Hence, the lack of a sufficiently deep mathematical analysis of these disturbances and the related distortions permits neither reliably estimate the level of their influence and importance in comparison with other sources of distortions of DAC output spectrum, nor to compare the existing methods for distortion reduction, nor give numerical recommendations for the practical synthesis in the step of DAC circuit parametric optimization.
The paper discusses the mentioned distortions in de-tails. A mathematical model is proposed describing the disturbances occurrence in the common biasing circuits and the distortions caused by them. Analysis of the model allowed to determine parameters of spur components in the DAC output signal spectrum and to obtain expressions that describe SFDR over the entire range of operating frequencies (first Nyquist band).
The proposed model and SFDR expressions are of great practical importance, as they allow to determine numerically the acceptable level of disturbances in the common biasing circuits and to implement directed parametric optimization of DAC circuit, for example, in the determination of the values of additional filtering capacitances in common biasing circuits. The model and the expressions in a common view can be applied to any common biasing circuit of current cells.
The adequacy of the proposed model and the obtained expressions is confirmed by simulation results.
Numerical examples illustrate that the disturbances in common biasing circuits can significantly degrade SFDR. However, sufficient reduction of their impact is not of significant complexity and can be realized at a slight increase in chip area and without increasing the power consumption.
 Current-steering, SFDR, spectrum distortions.
Library reference
 Svizev G.A., Ionov L.P., Mukhin I.I., Zolotarev A.V. Analysis of Distortions Caused by Impulses in Switching Transistors Sources in Current-Steering DAC // Problems of Perspective Micro- and Nanoelectronic Systems Development - 2016. Proceedings / edited by A. Stempkovsky, Moscow, IPPM RAS, 2016. Part 3. P. 57-64.
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