The delta–sigma modulator, a single-bit oversampled analog signal encoder,
was patented by Cutler in 1960 and described in the open literature by Inose
and Yasuda in 1962. It took until the mid-1980s, specifically the publication
of Candy’s widely-cited 1985 paper about the double integration modulator,
for the popularity of these modulators to take off. Since then, almost fifteen
years later, delta–sigma modulators have cornered the market in digital audio
converters (both analog-to-digital and digital-to-analog), and they are also used
to great advantage in lower-rate but still higher-resolution applications such as
instrumentation and seismometers.
Most modulators are implemented with a discrete-time loop filter using
circuit techniques such as switched-capacitor, likely because it is easy to map
the mathematics of modulators onto the implementation. Due to settling-time
constraints in typical discrete-time implementations, the maximum clock rate
is limited, and because delta–sigma circuits are necessarily oversampled, highresolution converters with only relatively narrow conversion bandwidths (up to
a few hundred kilohertz) can be built. Ideally, resolution is determined only
by the modulator loop filter and the oversampling ratio; if it were possible to
relax the clock rate restrictions and thereby increase the clock rate, then for a
given oversampling ratio and loop filter, high-resolution converters with wider
bandwidths (into the megahertz) could be built.
One way to achieve higher clock rates is to build the loop filter portion out of
continuous-time circuits such as transconductors and integrators. The idea of
using continuous-time filters in delta–sigma modulators is not particularly new:
Adams’ seminal 1986 paper describes one of the first functional delta–sigma
audio encoders that had a continuous-time loop filter built out of discrete op
amps, resistors, and capacitors. However, since the mid-1990s, semiconductor
processes have increased in speed to the point that it becomes possible to
integrate gigahertz-speed modulators. An example of an application where
such modulators are especially appealing is cellular radi an entire cellular
band (tens of megahertz) could be digitized, possibly at the IF or RF, and the
signal details sorted out with digital signal processing.