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20-Bit -Colinear- DAC, a Solution to Low Level Problems
Digital audio systems have traditionally used digital to analog converters (DACs) that employ laser trimmed, current sources in order to achieve sufficient accuracy. However, even the best of these suffer from potential low level nonlinearity due to errors at the major carry transition. More recently, DACs that utilize noise shaping techniques and very high oversampling frequencies have been introduced. These DACs do overcome the low level nonlinearity problem, but at the expense of signal to noise performance and often at the expense of channel separation and intermodulation distortion if the succeeding circuitry is not carefully designed. A new solution is required, one that combines all the advantages of a conventional DAC: excellent Full Scale performance, high signal to noise ratio and ease of use with excellent low level performance. Two DACs used in a complementary arrangement provide an ideal solution. The circuit employs two DACs that share a common reference and even a common R-2R ladder network to ensure perfect tracking under all conditions. By physically interleaving the individual bits of each DAC and employing laser trimming the highly accurate match required between DACs is achieved. This new complementary linear, Colinear, approach, which steps away from zero with small steps in both directions, avoids any glitching or large linearity error and also provides an accurate current output. The low level performance of this part is such that a 20 bit converter actually has validity.
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