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Shin Yokohama, Japan
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Ram,
Using the FFT a good method to non-linearity of a curent -switched D/A Converter. One comment would be that depending on the input frequency[from the digital code] and the clock rate, compensation may be necesary. A DAC acts like a zero-hold circuit, the frequency response includes a sinc function. So as the input frequency approaches the clock rate, the magnitude of the DAC output is reduced. More simply for an DAC to get a constant output level, the input level needs to increase as a function of frequency to cancel the effect of the sinc function.
Things to improve linearity are to increase the output resistance of the current source cell, minimize capacitance of the current source cell, minimize capacitance on the output node. Typically in current-switched designs, the code-dependent value of the DAC output impedance is a major source of non-linearity. Increasing the impedance reduces the effect of the "code-dependence", i.e., the DAC output is more "linear".
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