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Analysis of Distortions Caused by Output Capacitance Modulation in Current-Steering DAC
Authors
	Svizev G.A.
Date of publication
	2016
Abstract
	Basic elements of current-steering DAC architecture are current cells that compose the bits of the DAC. Output resistance and capacitance of current cell are different in on- and off-states. Therefore the output impedance of the DAC changes (is modulated) because of current cells switching. This modulation causes non-linear distortions of the output signal. For sufficiently low-frequency signals the output impedance of the DAC can be approximated by its output resistance. Thus non-linear distortions of the output signal are determined by the conversion static non-linearity, arising from output resistance modulation correlated with the input code of the DAC. However, by increase of output signals frequency the capacitive component of the DAC output impedance decreases and at sufficiently high frequencies plays the dominant role in determining the output impedance. Therefore, an additional analysis of its influence on output signal distortions is required. At the same time parasitic capacitances of transistors are mode-dependent values and can change significantly in large signal mode. For mentioned reasons, a parasitic modulation of DAC output capacitance arises. In conjunction with output resistance modulation it causes modulation of DAC output impedance. In turn, the modulation of output impedance causes a corresponding modulation of output time constant. Therefore because of cells switching the (normalized) pulse shape of the transition process on the DAC output varies according to the current output time constant. Finally, the distortions of the output signal pulses shape indicate the presence of non-linear distortions, that create spurious components in the spectrum of the DAC output signal. The paper analyses the existing researches [1], [2], [3] on the DAC output capacitance modulation. Shown, that the output capacitance modulation model used in [1] and [3] takes into account only a modulation component caused by current cells switching on / off but does not take into account a modulation component caused by DAC output voltage changing. The given numerical example illustrate that this unaccounted component in a value can be comparable with the first component and therefore requires consideration to improve the accuracy of the model. The new model of DAC output capacitance modulation is proposed. It considers the both specified components. The analysis of the presented model by means of Volterra functional series expansion ([5], [6], [7]) allowed to determine the arising non-linear distortions and to derive expressions that describe SFDR in the whole range of operating frequencies 0 ÷ fclk/2, where fclk is the DAC sampling frequency. Shown, that in the frequency range 0 ÷ fclk/6 SFDR is determined by third-order harmonic distortions (HD3) and the frequency range fclk/6 ÷ fclk/2 – by the third-order intermodulation distortions (IMD3). The adequacy of the proposed model and the obtained expressions is confirmed by simulation results.
Keywords
	DAC, harmonic distortions, HD3, intermodulation distortions, IMD3, SFDR.
Library reference
	Svizev G.A. Analysis of Distortions Caused by Output Capacitance Modulation 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. 65-72.
URL of paper
	http://www.mes-conference.ru/data/year2016/pdf/D186.pdf