1. technology and computing
2. electronic components

IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 12, DECEMBER 1997
1979
A 1-V 1.8-MHz CMOS Switched-Opamp
SC Filter with Rail-to-Rail Output Swing
Andrea Baschirotto, Member, IEEE, and Rinaldo Castello, Senior Member, IEEE
Abstract—A low-voltage switched capacitor (SC) filter operated
from a single 1 V supply and realized in a standard 0.5-m
CMOS technology is presented. Proper operation is obtained
using the switched-opamp technique without any clock voltage
multiplier or low-threshold devices. This makes the circuit compatible with future deep submicrometer technology. As opposed
to previous switched-opamp implementations, the filter uses a
fully differential topology. This allows operation with a rail-rail
output swing and reduction of the number of opamps required
to build high-order infinite impulse response (IIR) filters. On
the other hand, a low-voltage common-mode feedback (CMFB)
circuit is required. In addition, the circuit uses an opamp which
is only partially turned off during the off phase. This enables
an increase in the maximum sampling frequency. The filter
implements a bandpass response (fs =fo = 4; Q = 7) and it has
been characterized with a 1.8 MHz sampling frequency. Its power
consumption is about 160 W. The filter is still fully functional
down to 0.9 V supply voltage.
Index Terms—Switched-capacitor filters.
I. INTRODUCTION
I
N the last years, the interest toward low-power, low-voltage
IC’s has consistently grown. This is primarily due to two
factors: the increasing importance of portable equipment in all
market segments [1] and the reduction of the supply voltage of
modern IC due to technology scaling. On the other hand, it is
mandatory to use the latest technology to be able to integrate
increasingly more complex systems on a single chip. In the
direction of single chip systems, CMOS technology appears
to be the most advantageous, especially in terms of cost when
a very large silicon area is needed. As a consequence, CMOS
circuits operating at lower and lower supply voltages will be
required as technology scaling progresses.
Switched capicitor (SC) techniques have been demonstrated
to be an efficient way of implementing analog functions in
CMOS technology. The elements required for the realization
of SC circuits are capacitors, switches, and opamps. Supply
voltage reduction does not strongly affect capacitor properties.
On the other hand, turning on and off MOS switches and
maintaining proper opamp operation is difficult at low supply
voltages. In fact, reducing the supply voltage correspondingly reduces the overdrive voltage of the MOS switches,
and the classical approach based on the use of transmission
Manuscript received April 16, 1997; revised July 17, 1997. This work was
supported in part by CNR within “Progetto coordinato sistemi microelettronici
a basso consumo per apparecchiature portatili.” The project has been developed within the ESPRIT TIBIA Project.
The authors are with the Dipartimento di Elettronica, Universit´a di Pavia,
1-27100 Pavia, Italy.
Publisher Item Identifier S 0018-9200(97)08798-2.
gate (complementary switches) is no more effective. Fig. 1(a)
shows the standard noninverting SC integrator embedded in
a closed-loop structure. Fig. 1(b) shows the conductance of
a transmission gate in its on-state when it is connected to a
for
V. As
is varied between zero and
voltage
, the conductance remains always larger than a minimum
value gds . Fig. 1(c) shows the switch conductance for
V. A critical voltage region centered around
for which both switches are not conducting is present [14].
However, with rail-to-rail output swing, the output of the
opamp crosses this critical region. It follows that any switch
connected to the output of the opamp [S1 in Fig. 1(a)] will
not operate properly. On the other hand, the correct operation
of all the other switches of Fig. 1(a) (S2, S3, and S4) can be
guaranteed by properly choosing the value of bias voltages
, , and . A possible solution to correctly turn on the
critical switch S1 consists of biasing the opamp output either
close to ground or to the positive supply. This, however, limits
the output swing to either one of the two regions shown
in Fig. 1(c). On the other hand, at low supply voltage it is
mandatory to maximize the output swing in order to achieve a
sufficient dynamic range. This is because reducing the supply
voltage leaves the noise constant, while the signal swing is
reduced more than linearly with the supply. As a consequence,
ways to properly operate the switches while maintaining railto-rail output swing must be developed.
Three solutions to this problem have been proposed: the
use of lower threshold voltage devices [3], [4], the use of
an on-chip clock voltage multiplier [5]–[9], and the switchedopamp technique [10]–[13]. The use of low-threshold devices
is a high-cost solution since it requires a special technology.
Voltage multipliers, which generate clock phases higher than
the supply voltage to drive critical switches, cannot be used
in scaled-down technologies. The most attractive development
up to now appears to be the switched-opamp technique.
A noninverting switched-opamp SC integrator driven by
a similar stage is shown in Fig. 2 [10]. The critical switch
S1 is eliminated and its function is realized by turning on
and off the opamp through switch Sa. When the integrator
is used in a closed-loop structure and for zero input signal,
steady state is reached when no charge injection in virtual
ground occurs. Thus, the relationship
must hold. As a consequence, a compromise between the
achievable signal swing and the amount of overdrive voltage
of the noncritical switches must be reached. The original
V and
design [10] uses
mV, with a threshold
0018–9200/97$10.00  1997 IEEE
1980
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 12, DECEMBER 1997
(a)
Fig. 2. Original switched-opamp integrator [10].
The paper is organized as follows. In Section II, the
switched-opamp technique is discussed, and the improved
version is presented in detail. In Section III, the opamp is
described, emphasizing the solutions adopted to improve the
turn-on time and to implement a common-mode feedback
(CMFB) circuit compatible with the minimum supply voltage.
In Section IV, the experimental results relative to a prototype
filter which implements the proposed solutions are given.
Section V draws some conclusions.
(b)
(c)
Fig. 1. (a) Standard SC integrator, (b) switch conductance with VDD = 5 V,
and (c) switch conductance with VDD = 1 V.
voltage of 0.9 V. This solution gives an output swing of
550 mV that is about half of a true rail-to-rail solution.
In addition, minimum supply voltage required is given by
. This is much larger
than the minimum required by an optimized opamp.
In this paper, a modified version of the switched-opamp
technique is presented. Improvements are proposed both at the
system level and at the circuit level. At the system level, the
SC integrator of Fig. 2 is modified in order to independently
set the common-mode input voltages and the quiescent output
voltage of the opamp. In this way, true rail-to-rail operation
is guaranteed, and the minimum supply voltage becomes a
threshold voltage plus just two overdrive voltages. In addition,
a fully differential solution is adopted. At the circuit level, the
opamp is modified to reduce its turn-on time. This allows the
use of a higher sampling frequency.
II. THE IMPROVED SWITCHED-OPAMP TECHNIQUE
The original switched-opamp technique shows the following
limitations.
1) The minimum supply voltage required for proper operation of the filter is given by
[10]. In addition, for this supply
voltage, the available signal swing is zero.
2) The opamp is completely turned off during one phase
of the clock. This causes a long turn-on time during
the other phase which limits the maximum sampling
frequency.
3) A single-ended structure is used, allowing only noninverting integrators. Thus, extra inverting stages are
needed to build high-order infinite impulse response
(IIR) filters.
In this work, solutions for all the above limitations are
proposed as outlined below.
1) The minimum supply required is set by the opamp and
is equal to
. Furthermore, in any
condition, a rail-to-rail output swing is obtained [12].
2) The opamp uses a two-stage topology where only the
output stage is turned on and off and the charge stored
in the Miller capacitance is maintained during the off
phase. This allows the use of a higher sampling frequency.
3) A fully differential topology is used. This gives, at no
extra cost, the sign change needed to build high-order
IIR filters.
Fig. 3 shows the single-ended version of the proposed
switched-opamp SC integrator structure. A voltage level shift
between the opamp input common-mode voltage and the
output quiescent dc-voltage of the opamp is implemented by
switching
between
and ground and injecting a fixed
amount of charge in the virtual ground.
BASCHIROTTO AND CASTELLO: 1-V 1.8-MHz CMOS SWITCHED-OPAMP SC FILTER
1981
(a)
Fig. 3. Improved switched-opamp integrator.
X
(b)
during operation. (b) Continuous time
Fig. 5. (a) Excursions of node
analysis of the integrator after turn on edge.
Fig. 4. Output swing comparison for the two integrators: line I—output
swing for the proposed switched-opamp integrator (Fig. 3) and line II—output
swing for the original switched-opamp integrator (Fig. 2).
In steady-state condition (i.e., no charge injection in virtual
ground) the following relationship must hold:
. Choosing
, and
to ground,
results in equality to
,
setting
ensuring rail-to-rail output swing. This allows the simultaneous
optimization of switch operation and output swing.
All switches are connected either to ground (and realized
(and realized with PMOS). Thus, the
with NMOS) or to
minimum supply voltage required for the switch operation is
, where
is the larger of the two
threshold voltages (N-type and P-type). Notice that this is the
same as the minimum supply voltage required for the operation
of digital CMOS circuits.
The difference between the original and the new circuit in
terms of available output swing can be quantitatively estimated
with a numerical example using typical technology and design
V,
V,
.
parameters, i.e.,
for
Fig. 4 plots the maximum swing achievable versus
the two cases. For the new approach (line I), proper operation
equal to 0.85 V compared to a value of about
starts at
1 V for the original one (line II). Furthermore, while for
the new approach at
V the available swing is
already 0.65 V (i.e., rail-to-rail), for the original approach
V the swing is still zero. This is obtained at the
at
cost of a 50% increase in the output noise power due to the
.
additional SC branch of
A potential problem using
equal to ground is the
possible charge loss during the turn on transients. In fact, the
inverting opamp input node is always connected to a reverse
biased pn junction associated with the drain/source of the
NMOS switch S4, as shown in Fig. 5(a). This bulk diode
could be forward biased by sufficiently large negative voltage
spikes caused by the charge injection and the finite speed of
the opamp. Simulations show that for a 500-mV spike of 5
ns duration and assuming a minimum size junction, a 0.5-mV
voltage error on a 1-pF integration capacitor at 100 C results.
At the turn-on instant (while the opamp is not active due
to its finite bandwidth) two voltage steps are applied to
the injecting capacitors
and
, as shown Fig. 5(b).
Capacitor
is driven with a negative step (it is switched
from
to ground) and tends to decrease the voltage at
, while capacitor
is driven with a positive step (it is
switched from
to
) and tends to increase the voltage at
. The negative spike can then be reduced ensuring that the
positive contribution from
is injected before the negative
contribution from
. This can be done in two ways: with
a proper sizing of the switches and with the use of delayed
clock phases [15].
Assuming that the two capacitors are connected simultaneously, the excursion of the voltage
can be described in
the time domain by the equation
(1)
where
and
connected to
and
are the impedance of the switches
, respectively. Therefore, choosing
1982
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 12, DECEMBER 1997
Fig. 6. Complete low-voltage switchable opamp.
the size of the switches in such a way that
can always be made to move in the positive direction. Notice
that the use of large size for these switches is not critical for
the charge injection because they are not connected to the
virtual ground.
This approach can be improved by the use of delayed clock
phases, usually adopted to reduce signal dependent distortion
[15]. In fact, in addition to the longer time constant associated
with
, this capacitor is also driven with delayed phase
1 Nd.
A further problem with the structure of Fig. 3 is the disturbance coupled into the circuit due to
. An error in
the
size results in an extra offset, while all the noise
present on
is injected into the signal path. This problem
can be greatly alleviated using a fully differential structure. In
this case, any disturbance (offset or noise) injected by
results in a common-mode signal which is rejected by the
fully differential operation. The amount of the cancellation is
limited by the mismatch of the two differential paths.
III. LOW VOLTAGE OPAMP DESIGN
At low supply voltage, stacked configurations (like cascode)
must be avoided. As a consequence, multistage structures are
used to achieve a sufficiently high dc-gain. In this design, the
input and the output common-mode voltages of the opamp can
be independently set. This allows us to independently design
and separately optimize the input and the output stage.
The complete two-stage switched-opamp topology is given
in Fig. 6. The key point for the output stage is to achieve a
rail-to-rail swing. As a consequence, a very simple structure
is used with no cascode in the signal path (M9, M10, and
switch M11). It can operate with a minimum supply voltage
of
.
The input stage is the most critical block when the supply
voltage is reduced. For a common-mode input voltage
equal to ground, a p-type differential input stage is used (M1,
M2, M3, M4, and M5). The signal path is then folded (M6,
M8) to ensure proper operation at low supply-voltage [2]. The
minimum supply voltage
for the input stage is given
by
(2)
where the first condition must be satisfied to guarantee that
the three stacked devices M1, M3, M5 operate in saturation
region (i.e.,
). If
is larger than
, as is usually
the case, the second term dominates and the minimum supply
. This is
voltage required by the input stage is
the same voltage required by the output stage.
The key target in the opamp design is to minimize its turn on
time, since this limits the sampling frequency. In this direction,
only the second stage is turned off acting on M11, while the
first stage is kept active even during the off phase.
is connected to the source of M6
The Miller capacitor
voltage
through switch MS. Placing the source of M6 one
higher than ground allows the proper operation of switch
MS with the same
required by the opamp. When
the output stage is turned off, MS is opened disconnecting
from M6. The charge stored on
at the end of the
active phase is conserved and no charging delay occurs at the
beginning of the next active phase.
The fully differential structure requires a CMFB circuit
able to operate in the presence of a large opamp output
swing. This circuit is shown in Fig. 7. It is a modified
version of the dynamic CMFB used in many classic fully
differential amplifiers [8]. The opamp used in the CMFB
circuit is a single-ended structure with PMOS input stage
and input common-mode voltage equal to ground. During
and
phase 1, the main opamp is off, and capacitors
(
pF) are charged to
, and capacitor
CCM is completely discharged. In the same phase, capacitors
pF) are disconnected and remain charged. In
CFF (CFF
this way, capacitors CFF implement a zero which improves
stability of the common-mode control loop. During phase 2,
the loop is closed and all capacitors inject charge into CCMFB
pF). The CMFB circuit steady state occurs
(CCMFB
when during phase 2 no charge is injected in the integration
capacitance. This charge is given by
. Therefore, with
(i.e.,
pF) the output opamp common mode is set
.
to
Table I gives the size of the devices in the opamp and
Table II summarizes the opamp performance.
The opamp bandwidth is such that for a feedback factor
, a sampling frequency larger than 10 MHz should
be possible, within a standard SC structure. However, in a
BASCHIROTTO AND CASTELLO: 1-V 1.8-MHz CMOS SWITCHED-OPAMP SC FILTER
1983
Fig. 7. Low-voltage sampled-data CMFB circuit.
Fig. 8. Prototype bandpass filter architecture.
TABLE I
OPAMP DEVICE SIZES
TABLE II
OPAMP SIMULATED PERFORMANCE
switched-opamp solution, the settling time is increased by the
delay required to wake up the opamp (about 75 ns at 1 V
supply). This delay limits the maximum sampling frequency
to about 7 MHz.
metal levels and two poly layers which are used to implement
all the capacitors. The filter is a biquad cell implementing
a bandpass response with nominal center frequency equal to
one fourth of the sampling frequency and nominal
.
The filter structure is shown in Fig. 8. A unit capacitance
of 0.3 pF has been used and the capacitor values are the
following:
pF,
pF,
pF,
pF,
pF,
pF,
pF,
pF. In a bandpass filter it is possible to
IV. FILTER PROTOTYPE AND EXPERIMENTAL RESULTS
Using the proposed solutions, a prototype filter has been
realized within a 0.5- m CMOS technology featuring three
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 12, DECEMBER 1997
Fig. 10. Frequency response (VDD = 1 V).
Fig. 9. Chip photograph.
use a capacitive input coupling
. For a switched-opamp
structure this is an advantage with respect to low-pass filters
which require an input dc-coupling. The chip photograph is
shown in Fig. 9. The chip area is about 0.15 mm .
In a bandpass filter with
( is the sampling
frequency,
is the bandpass center frequency), a very large
slew rate is required since output step as large as half the signal
amplitude are present. This is not the case for a classical lowpass filter where
is generally much larger than four. Due
to this requirement, the filter performance (maximum clock
frequency, linearity, etc.) depends on the amplitude of the input
signal. This effect is shown in the following.
All measurements were done with
V. The filter
consumes about 160 W. Fig. 10 shows the filter frequency
response at different clock frequencies from 1.8 MHz to
peaking (2 dB) results.
4.2 MHz. Up to 3.4 MHz, a small
Beyond this sampling frequency, the peaking becomes more
pronounced due to incomplete opamp settling. The following
measurements were done with
MHz. The total output
noise is about 800 V . The immunity with respect to noise
on the supply has been measured in terms of power supply
rejection ratio (PSRR) in the passband, and in terms of power
supply rejection (PSR) up to
. The PSRR in the band
300 kHz–600 kHz is shown in Fig. 11. A PSRR better than
46 dB is measured. In addition, for frequency up to
, the
PSR always stays lower than 45 dB. These measurements
demonstrate the validity of the differential solution which
guarantees immunity with respect to noise on the supply, even
if it is directly injected in the signal path.
Fig. 12(a) and (b) shows the single-ended and the differential output signals, respectively. It can be see that the signal
is settling even for a signal amplitude as large as 1.6 V .
In this case (since the output common mode is set slightly
higher than
), the peak signal is within 100 mV from
the supply voltage. From these waveforms, it is evident that the
switched opamp mechanism is based on the fact that during
one clock phase the output signal is not available and the
output node is tied to the power supply (return-to-zero). This
Fig. 11. PSSR measurement.
fact must be taken into account in the overall signal processing
system embedding the switched-opamp filter. If the signal
is processed in a continuous-time manner, the return-to-zero
results in a gain loss of 1/2 and in a linearity degradation, due
to the large output steps resulting in slew rate and glitches.
These phenomena degrade the linearity of the continuous-time
waveform, but they have no effect on the sampled version
of the output signal and then they can be avoided with a
signal resampling, as occurs when the switched-opamp filter
is used in front of an analog-to-digital converter (ADC). In
the following, linearity measurements have been performed
extensively for the continuous-time output waveform, and for
a specific case they have been compared to those obtained
using only the output samples.
For the overall continuous-time waveform, a 3% intermodulation (IM) is measured for two input signals of 500 mV .
On the other hand, the 1% IM corresponds to two 400 mV
input signals as shown in Fig. 13. The dynamic range (defined as signal /noise ) for 3% IM is about 52 dB. The
total harmonic distortion (THD) is measured with an input
frequency at
kHz. The third harmonic at
is
BASCHIROTTO AND CASTELLO: 1-V 1.8-MHz CMOS SWITCHED-OPAMP SC FILTER
1985
Fig. 13. 1% IM measurement.
(a)
Fig. 14. 1% THD measurement.
TABLE III
FILTER PERFORMANCE
(b)
Fig. 12. (a) Output single-ended waveform with 1.6 Vpp differential input
signal. (b) Output differential waveform with 1.6 Vpp differential input signal.
folded at
which for the specific bandpass filter
(with
) is in the passband. The 1% THD corresponds
to a 725 mV input signal and the 3% THD to a 1.1 V
input signal, as shown in Fig. 14.
For the case of two 700 mV signals, the output signal has
been sampled and a fast Fourier transform (FFT) analysis has
been performed. A 1.8% IM has been measured, while for the
continuous-time waveform the IM is 4%.
Table III summarizes the filter performance.
Finally, the filter is still fully functional with a supply
voltage as low as 0.9 V. Fig. 15(a) shows the frequency
response for sampling frequency between 500 kHz and 3 MHz
with 500 kHz step. On the other hand, for
MHz, a
higher sampling frequency can be used. Fig. 15(b) shows the
frequency response with sampling frequency between 1 MHz
and 9 MHz with 1 MHz step.
0.5-m CMOS
1V
160 W
1.8 MHz
6.6
435 kHz
1.6 Vpp
800 Vrms
725 mVpp
1.1 Vpp
800 mVpp
1 Vpp
52 dB
-46 dB
0.15 mm2
Technology
Supply voltage
Power consumption
Sampling frequency
Q
fo
Max. output swing
Total output noise
THD 1%
THD 3%
IM3 1%
IM3 3%
Dynamic range (IM3 3%)
PSRR
Filter chip area
V. CONCLUSION
The possibility of realizing SC circuits operated down to
1 V supply in a standard 0.5- m CMOS technology with
sampling frequency in the MHz range is demonstrated. The
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 12, DECEMBER 1997
ACKNOWLEDGMENT
The authors wish to thank F. Montecchi (University of
Pavia) and A. Nagari (ST Microelectronics) for helpful suggestions and discussions.
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16
(b)
Fig. 15. (a) Frequency response with VDD
: V.
with VDD
= 12
= 0:9 V. (b) Frequency response
circuit is implemented with the switched-opamp technique,
thus it does not require any clock voltage multiplier. Several
new techniques have been proposed in order to solve critical
limitations present in the previous switched-opamp circuits. In
particular, the paper has presented a fully differential structure
that improves dynamic range (rail-to-rail output swing is
achieved) and an optimized opamp design that increases the
possible sampling frequency. This design demonstrates that
it is possible to achieve, with the SC filter operating at
1 V, performance comparable to that achieved with circuits
operating at higher supply voltages.
Andrea Baschirotto (S’89–M’95), for photograph and biography, see p. 932
of the July 1997 issue of this JOURNAL.
Rinaldo Castello (S’78–M’87–SM’92), for photograph and biography, see p.
932 of the July 1997 issue of this JOURNAL.