Texas Instruments SLAU039, User Manual

Page: 15 / 66

Special Considerations When Using Sigma-Delta Converters
1-5
Introduction
1.3.2 Disadvantages
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The digital filters in sigma-delta converters introduce into the conversion
process a time delay that makes them unsuitable for some control applica-
tions. The AD50 ADC has a delay of 17 samples and the DAC has an 18
sample delay.
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It is not practical to multiplex several inputs to a single sigma-delta ADC
except at very low rates, because each channel would be corrupted by the
earlier samples from other channels still propagating through the digital
filter. Although the AD50 has an input multiplexer, this is only for selecting
one or another input, not for interleaving two input channels onto one data
stream.
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Audio band converters are optimized for ac signals and a small dc offset
may be present.
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Spurious low level tones can sometimes occur at very low input signal
levels, especially if clock signals at Fs/2 are allowed to couple into the
reference voltage pins. Such tones can be identified because their
frequency is affected by small changes in dc offset.