1063-7397/03/3206- $25.00 © 2003 MAIK “Nauka /Interperiodica”
Russian Microelectronics, Vol. 32, No. 6, 2003, pp. 333–338. Translated from Mikroelektronika, Vol. 32, No. 6, 2003, pp. 414–420.
Original Russian Text Copyright © 2003 by Rogatkin.
-modulation analog-to-digital converters (ADCs)
essentially include an operational ampliﬁer (op amp)
that serves as an integrator. Modern high-resolution
designs for the audiofrequency range may have ﬁve or
more integrators, depending on the order of the modu-
lator . The feedback capacitor of such an op amp is
usually a ﬁxed parallel-plate capacitor realized as a
bilayer consisting of a polysilicon gate region and
another, specially produced polysilicon layer. Increas-
ing the product cost, this approach appears to be
impractical when the word length required is less than
14–16 bits or the ADC is a minor component of the
One alternative is to employ the metal–oxide–semi-
conductor (MOS) capacitor formed by the gate region
and interconnected source and drain regions of a MOS
transistor, with a proper bias applied to the substrate.
An advantage of such capacitors is that they can be
made by standard complementary MOS (CMOS) tech-
nology. On the other hand, the capacitance is essen-
tially dependent on applied voltage if this is too low.
The operating voltage range of a MOS capacitor is
divided by the threshold voltage of the transistor. The
behavior of the capacitance changes abruptly when
crossing this level. At lower voltages the MOS capaci-
tance is determined by the gate–source, gate–drain, and
gate–substrate capacitances. At higher voltages the
gate–channel capacitance plays a major part. This volt-
age region should be used for operation.
Figure 1 contains computed graphs of different
capacitances against the substrate bias with respect to
the source–drain electrode. The calculation is made
with the Cadence Spectre software for an n-channel
transistor of area 10
and channel length
m (the minimum value).
CMOS Switched-Capacitor Integrator
Yu. B. Rogatkin
Moscow Institute of Engineering Physics (Technical University), Moscow, Russia
Received November 15, 2002
—A switched-capacitor integrator is designed, and its performance is evaluated by computer simula-
tion. The integrator is intended to operate in
-modulation ADCs realized in basic CMOS technology. A cir-
cuit diagram of the integrator is shown. The results of a transient analysis are presented.
2 3 4 5 6
Computed graphs of different capacitances against the substrate bias with respect to the source–drain electrode. The curves
) source–drain, (
) source–channel, and (
) gate–channel capacitance. Curves
are plotted for below-threshold
corresponds to above-threshold biases.