17-03-2014, 08:02 PM
technique is proposed to adjust
coefficients of transfer functions in switched-capacitor
circuits. Transfer-function-coefficients are defined by
capacitor-values considering full-charge transfer
among capacitors. The proposed tuning technique in
this paper is based on adjusting the amount of charge
transferred from one capacitor to next capacitor. By
all means, the net charge transferred in switchedcapacitor
circuits will effectively modify transfer
function of a particular block without modifying
individual capacitor values.
Index Terms — Switched capacitor, tuning, charge,
transfer function, settling, switch, on-resistance
1. Introduction
Switched-capacitor (SC) circuit is one of the most
common design blocks in modern Integrated-Circuit
(IC) market due to its high accuracy, relative-mismatch
dependency (based on ratio of capacitors), and
compatibility of modern submicron CMOS process [1].
These circuits are baseline for analog-to-digital,
digital-to-analog converters, signal conditioning
circuits, and filters, etc [2].
SC circuits offer many advantages such as high
linearity, low-noise, and well-defined transfer function
[3]. Often, transfer-function-properties such as; corner
frequencies of low-pass and high-pass filters or center
frequencies of band-pass filters need to be controlled
or programmed to different values [4-7]. This initiates
designs with tunable gain factors, which introduce
great design flexibility. At the same time, due to
silicon-cost of additional elements for tuning, new
design techniques, with compact layout-area, are
highly desirable.
Normally, Circuit techniques have been to use
multiple capacitors or to use multiple clock phases in
order to provide this extra design freedom. Adding or
modifying number of capacitors will easily change the
coefficients of transfer functions. Hence, this method
allows designing adaptive switched-capacitor filters.
On the other hand, it will directly increase the chip
area significantly. Using multiple clock phases is a
variation of the first technique which also enables
programmability. However, this option adds digital
circuits and extra clock phases, which will not only
increase substrate noise but also need careful layout
design and clock routing.
Basic idea in this paper is to control net charge on
capacitors [8]. In fact, considering charge-voltage
relation (Q = V * C); one variable is capacitor value
and the other variable is charge amount.
Programmability can be provided either changing
capacitor values or the net charge transferred. In this
paper, a method is described to control amount of the
charge in order to change transfer functions of
switched capacitor circuits. There is also similar
research focused on adjusting the charge transferred
based on the duty-cycle ratio of clock phases [9-10]. In
our approach, charge is controlled by using on
resistance of switches. This resistance is controlled by
changing the gate voltages of switches.