Among analog-to-digital converters, the delta-sigma modulator has
cornered the market on high to very high resolution converters at
moderate speeds, with typical applications such as digital audio and
instrumentation. Interest has recently increased in delta-sigma circuits
built with a continuous-time loop filter rather than the more common
switched-capacitor approach. Continuous-time delta-sigma modulators
offer less noisy virtual ground nodes at the input, inherent protection
against signal aliasing, and the potential to use a physical rather than
an electrical integrator in the first stage for novel applications like
accelerometers and magnetic flux sensors. More significantly, they relax
settling time restrictions so that modulator clock rates can be raised.
This opens the possibility of wideband (1 MHz or more) converters,
possibly for use in radio applications at an intermediate frequency so
that one or more stages of mixing might be done in the digital domain.
Continuous-Time Delta-Sigma Modulators for High-Speed A/D Conversion:
Theory, Practice and Fundamental Performance Limits covers all aspects
of continuous-time delta-sigma modulator design, with particular
emphasis on design for high clock speeds. The authors explain the ideal
design of such modulators in terms of the well-understood discrete-time
modulator design problem and provide design examples in Matlab. They
also cover commonly-encountered non-idealities in continuous-time
modulators and how they degrade performance, plus a wealth of material
on the main problems (feedback path delays, clock jitter, and quantizer
metastability) in very high-speed designs and how to avoid them. They
also give a concrete design procedure for a real high-speed circuit
which illustrates the tradeoffs in the selection of key parameters.
Detailed circuit diagrams, simulation results and test results for an
integrated continuous-time 4 GHz band-pass modulator for A/D conversion
of 1 GHz analog signals are also presented.
Continuous-Time Delta-Sigma Modulators for High-Speed A/D Conversion:
Theory, Practice and Fundamental Performance Limits concludes with some
promising modulator architectures and a list of the challenges that
remain in this exciting field.
