Download An Integrated Full-Wave CMOS Rectifier With Built

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IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 55, NO. 10, NOVEMBER 2008
An Integrated Full-Wave CMOS Rectifier With
Built-In Back Telemetry for RFID and
Implantable Biomedical Applications
Maysam Ghovanloo, Member, IEEE, and Suresh Atluri, Member, IEEE
Abstract—This paper describes the design and implementation
of an integrated full-wave standard CMOS rectifier with built-in
passive back telemetry mechanism for radio frequency identification (RFID) and implantable biomedical device applications. The
new rectifier eliminates the need for additional large switches for
load modulation and provides more flexibility in choosing the most
appropriate load shift keying (LSK) mechanism through shorting
and/or opening the transponder coil for any certain application.
The results are a more robust back telemetry link, improved read
range, higher back telemetry data rate, reduced rectifier dropout
voltage, and saving in chip area compared to the traditional topologies. A prototype version of the new rectifier is implemented in the
m n-well 3-metal 2-poly 5 V standard CMOS process,
AMI 0 5
occupying 0 25 mm2 of chip area. The prototype rectifier was
powered through a wireless inductive link and proved to be fully
functional in its three modes of operation: rectification, open coil
(OC), and short coil (SC).
Index Terms—Back telemetry, CMOS, full-wave rectifier, implantable biomedical devices, inductive coupling, load shift keying,
radio frequency identification (RFID), wireless.
I. INTRODUCTION
R
ADIO FREQUENCY identification (RFID) systems
and implantable microelectronic devices (IMD) are two
major categories of devices that inductively communicate with
a reader over a short distance through a pair of loosely coupled
coils, which constitute a transformer [1]–[11]. Contactless data
transmission from the data-carrying device, also known as
transponder, to the reader is one of the key functions, called
back telemetry. It is also used in wireless sensing technology,
where direct electrical contact to the sensing element is not
feasible such as in tire pressure monitoring [12]. What is
in common in these applications is the extreme limitation
in size, which eliminates the use of a power source within
the transponder especially when combined with the need for
long term usage. A viable solution is to inductively power up
Manuscript received May 7, 2007; revised October 5, 2007. First published
April 18, 2008; current version published November 21, 2008. This work was
supported in part by the Department of Electrical and Computer Engineering,
North California State University, Raleigh, NC 27695 USA. This paper was
recommended by Associate Editor P. Heydari.
M. Ghovanloo is with the GT-Bionics Lab, School of Electrical and Computer
Engineering, Georgia Institute of Technology, Atlanta, GA 30308 USA (e-mail:
[email protected]).
S. Atluri is with Integrated Device Technologies Inc., San Jose, CA 95138
USA (e-mail: [email protected]).
Digital Object Identifier 10.1109/TCSI.2008.924877
Fig. 1. Simplified block diagram of an inductively powered RFID or implantable microelectronic device (IMD).
the device via the same telemetry link that is setup for data
transmission. As a result, the data communication and power
handling blocks in RFID/IMD systems are highly intertwined
and should be designed and developed together [1].
Fig. 1 shows a simplified block diagram of an RFID/IMD
system. Power and data transmission take place across the
and secondary
inductive link formed by the primary
coils on the reader and transponder sides, respectively.
by a large sinusoidal signal,
A power amplifier (PA) drives
known as power carrier, at specific frequencies
dedicated
to RFID/IMD applications in the Industrial-Scientific-Medical
and a series capacitor,
, can form an
(ISM) band [13].
oscillating tank circuit and become part of a class-E PA, which
is a popular topology for RFID/IMD applications since it can
theoretically reach power efficiencies up to 100% [14], [15].
It is possible to modulate the amplitude, frequency, or phase
of the power carrier by applying a modulating signal (MOD)
to the PA and detecting these changes on the transponder side.
Therefore, MOD can be used to transmit commands along with
power from the reader to transponder, a function that is usually
referred to as forward telemetry [16].
Only that part of the electromagnetic flux generated by
which passes through
can participate in inducing current in
and powering the transponder. This is characterized by the
coils mutual coupling , which is dependant on the coils geometry, relative distance, orientation, and magnetic properties
of the medium (air, tissue, or ferrite core) [16]–[18]. A capacis usually added in parallel to
in order to form a secitor
ondary tank circuit, tuned at , in order to boost the received
voltage and improve the efficiency. To convert the received ac
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GHOVANLOO AND ATLURI: INTEGRATED FULL-WAVE CMOS RECTIFIER
power carrier to dc, the
thank circuit is followed by a rectifier, which is the focus of this paper.
Back telemetry can be performed in two ways: active and
passive. In active back telemetry, which is used when there
is a high volume of data to be sent to the reader at high rate,
the transponder is equipped with a transmitter and a dedicated
antenna operating at a frequency often much higher than
[20]. The high-power consumption of the transmitter and the
space needed for its circuitry and antenna, however, disfavor
active back telemetry in most applications where lower data
rates are acceptable. Passive back telemetry is possible by load
modulation, also known as load shift keying (LSK), which
is the most common technique for data transmission from
RFID/IMD transponders back to the readers by a large margin
[1], [10]. In this method, the loading of the transponder coil
in Fig. 1) is varied with the outgoing serial data stream,
(
due to the coils mutual
altering the impedance seen through
coupling, . The impedance change can either be resistive or
capacitive, resulting in amplitude shift keying (ASK) or phase
shift keying (PSK) of the back scattered signal [21]. In the rest
of this paper we will focus on the ASK.
It is desired to establish a robust back telemetry mechanism
that can handle a large enough distance between the reader
and transponder, known as the reading range or interrogation
zone. Depending on the application, a large reading range also
means that the system is robust enough to handle a certain
degree of coils misalignments, motion artifacts, nearby metallic
objects, and external electromagnetic noise and interference.
The dynamic reading range of the RFID/IMD systems depends
on detectability of the LSK impedance changes above the noise
and interference levels on the reader side. This in turn depends
, unloaded quality factors
on the coils mutual inductance
, and magnitude of the transponder load variations
as a result of load modulation. To achieve a large
reading range, we would like to compensate the effect of small
with the other parameters. In this paper, we assume that
and
, which are mainly determined by the coil designs, are
such
already optimized. Thus, our goal is to maximize
that it can be easily detected.
Load modulation in current designs depends on the nominal loading of the transponder coil. In low power RFID appliis large, a parallel MOS switch shorts
cations, where
ahead of a rectifier for a short period of time [1]. Alternatively,
a parallel switch can inflict impedance changes by altering the
configuration of the rectifier from full-wave to half-wave and
vice versa, as explained in [10]. In medium and high power
tends to be small, a series
RFID/IMD applications, where
MOS switch is being used after a full-wave rectifier to open the
could
load [11]. In complex RFID/IMD systems, however,
itself be a highly variable parameter depending on the state
of the transponder while being interrogated. For example, the
idle current consumption of a multichannel implantable microstimulating system, used in auditory or visual prostheses, might
be negligible compared to its current consumption when multiple stimulating channels are simultaneously active [22], [23].
alone may not provide a large
Therefore, shorting or opening
in all conditions, degrading the nominal reading
enough
range. Further, the load modulation MOS switches, which have
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Fig. 2. (a) Basic circuit model for the analysis of the inductive link and its back
telemetry mechanism. (b) Simplified model of the transponder circuit.
to be large to provide small “on” resistance, consume chip area
and in case of the series switch [11] result in additional dropout
voltage, degrading the overall system power efficiency.
We have developed an integrated full-wave rectifier in standard CMOS technology with built-in back telemetry, which can
, to produce a large
both short and open the transponder coil
regardless of the actual load variations. In addition, by
combining rectification and back telemetry functions in one circuit block (the gray box in Fig. 1); we have achieved significant
saving in chip area and eliminated the need for any additional
large switches before or after the rectifier. This can also reduce
the dropout voltage and improve the overall power efficiency.
In the following section, the theoretical basis for load modulation scheme has been laid out with emphasis on the parameters
that have the most significant effects on the ASK back telemetry
mechanism. Section III describes the circuit topology of the new
rectifier and its modes of operation. The simulation and experimental measurement results of a prototype rectifier with built-in
back telemetry are depicted in Section IV followed by a short
discussion and concluding remarks in Section V.
II. LOAD MODULATION FOR BACK TELEMETRY
Fig. 2(a) shows a circuit model of Fig. 1 block diagram that
helps analyzing the load modulation mechanism. The only independent power source in this circuit is a sinusoidal voltage
operating at , which represents the class-E power
source
amplifier with an output impedance of
. Current induces
across
that is shown as a separate
a voltage
current controlled voltage source, which drives the transponder
circuitry by creating .
To model the rectifier, which is a nonlinear circuit, we have
is much larger than
assumed that the load time constant
. This is usually the case in order to reject supply ripples
across the load. We replaced the rectifier with an ideal diode
in series with a dc voltage source equal to the rectifier dropout
. Therefore, the voltage across
would be a dc
voltage
, where
. We can then
voltage equal to
for the rectifier and
by
find an ac equivalent resistor
considering the amount of power consumed by
[5].
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(1)
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(2)
, the transponder
By replacing the load and rectifier with
simplifies to a series/parallel RLC circuit, shown in Fig. 2(b),
and loaded quality factor of
with a loop impedance of
(3)
(4)
is the parasitic resistance of the transponder coil [1].
where
The transponder reflected impedance onto the reader is shown
in series with
and its numerical value can be
in Fig. 2 as
found from [10]
Fig. 3. Reflected transponder impedance onto the reader side (Z ) versus
equivalent resistance seen across the rectifier input (R ). Other inductive link
parameters are listed in Table I.
(5)
where
is the parasitic resistance of
coupling coefficient, which is defined as
and
is the coils
TABLE I
SPECIFICATION OF THE INDUCTIVE LINK
(6)
In order to increase the interrogation zone,
variations as a
changes should be maximized. Equations (2)–(6)
result of
can provide significant insight in understanding which paramand consequently the interrogation
eters in Fig. 2(a) affect
zone. Some conclusions are following:
and dei) is the most significant factor that affects
pends on the coils distance, orientation, inductance, geometry, and magnetic properties of the medium [18]. The
reading range depends on the minimum for which the
variations.
transponder can produce detectable
and
need to be maximized by increasing
and
ii)
and decreasing
and . However, these parameters
are interrelated and there is usually a physical size conand
should
straint especially on . In addition,
with
and , respectively. Therefore,
resonate at
a compromise should be made to optimize these parameters with proper choice of the coils geometries, number
of turns, and wire thicknesses, which have been covered
in the literature extensively [5], [11], [16]. It should also
be noted that the efficiency of the inductive link, %, is
proportional to
, which is present in the numerator of (5) [5].
because it
iii) It would also be helpful to maximize
would increase , as shown in [1], and results in higher
efficiency, as well as improved back telemetry signal. By
according to (7), which is derived by difchoosing
at the resonance freferentiating (4) with respect to
can be found for every given
quency, the best value for
pair of
and
.
iv) Reducing the PA output resistance
similar to reducing
improves detectability of
variations.
v) Another important factor in extending the interrogation
, which along with
are the only paramezone is
ters in (5) that can be varied with the telemetry data to af. Fig. 3 shows how the magnitude of
changes
fect
when other inductive link parameters
with respect to
in (5) are selected from our experimental setup listed in
is a monotonic function of
in its
Table I. Since
is maxipractical range, the best way to maximize
, which is proportional to
according to
mizing
(2). Therefore, increasing the transponder load variations
improves the reading range in passive back telemetry.
as a
It is instructive to examine the two extreme limits of
result of load modulation when the
load is short or open cirin Fig. 2(b), the transponder circuit reduces
cuit. If we short
circuit and the reflected impedance can be found
to an
from
(7)
(8)
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GHOVANLOO AND ATLURI: INTEGRATED FULL-WAVE CMOS RECTIFIER
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TABLE II
RECTIFIER MODES OF OPERATION
Fig. 5. Chip micrograph including two rectifiers with built-in back telemetry
and a few on-chip ripple rejection capacitors implemented in AMI-0.5 m standard n-well CMOS process (Die size: 1.5 mm 1.5 mm).
2
Fig. 4. (a) Schematic diagram of the full-wave CMOS rectifier with built-in
back telemetry. (b) Schematic diagram of a 2:1 multiplexer (MUX) with dc
level shifter.
On the other hand, if
is opened, the transponder circuit in
Fig. 2(b) simplifies to an
circuit in which
and
cancel out at resonance frequency and
can be found from
(9)
and
for parameters listed
The numerical values of
in Table I are 1.39 and 7.99 , respectively.
III. BACK TELEMETRY RECTIFIER CIRCUIT DETAILS
Fig. 4 shows the schematic diagram of the new rectifier with
built-in back telemetry mechanism, which is an enhanced version of the CMOS rectifier that we reported in [19]. The recti, and
, and
fier consists of four main transistors:
a few smaller switches that control the operation of these large
current carrying transistors based on coil and output voltages.
and
directly connect to the
The rectifier input terminals
tank and the rectified dc output is delivered
transponder
to the load through
terminal.
and
are also connected
to a pair of high voltage protection circuits that consist of several nMOS transistors in series which start shunting current to
or
exceed a certain limit (
V). The
ground when
rectifier has three modes of operation that are controlled by its
Fig. 6. Measured rectifier operation in Mode-0 showing the waveforms at the
input terminals (V and V ), output terminal (Vout), and the separated n-well
), which closely follows Max(V ; V ; V
).
body voltages (V
Open Coil (OC) and Short Coil (SC) digital inputs. These operating modes are also summarized in Table II.
Rectifier: In this mode, which is
Mode-0
the default rectifier configuration, a pair of 2:1 multiplexers
and
(MUXp) controlled by OC connect the gates of
to
turning them into large (4 mm/0.6 m) diode-connected pMOS devices, which direct induced current from
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= 330 Fig. 7. (a) Measured back telemetry waveforms in Mode-1 using Open Coil (OC) input when R
. (b) Measured back telemetry waveforms in Mode-2
: k . From top: serial back telemetry data, secondary coil voltage, rectified output, and primary current through a
using Short Coil (SC) input when R
current sense transformer.
= 11 to the load. Meanwhile, another pair of 2:1 multiplexers
and
(MUXn) controlled by SC connects the gates of
(1.96 mm/0.6 m) to
and , respectively, to return the load
current back to the coil from the grounded p-type substrate.
and
which are parasitic diodes between p-sub and
and
drain terminals also help in returning current back to the coil
or
V. To eliminate latchup and substrate
when
and
are created on separated n-well regions
leakage,
(Body and Body ) and a pair of transistors
to
) is
added to each rectifying pMOS complex to dynamically control
and
). As a result
their isolated n-well potentials (
) and
)
[19]. This mechanism also minimizes the rectifier dropout
and
by eliminating their body effect,
voltage across
further improving the overall efficiency. Each MUX in Fig. 4(a)
is equipped with a cross coupled dc level shifter, shown in
Fig. 4(b), which converts the rectifier input logic levels from
to
so that they are applicable to the pass
gates.
Open Coil: In this mode MUXp
Mode-1
pair connects the gates of
and
to their isolated body terand
. Therefore,
and
minals resulting in
turn off and effectively detach the load from
.
and ,
which stay in the same state as in Mode-0, remain off because
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GHOVANLOO AND ATLURI: INTEGRATED FULL-WAVE CMOS RECTIFIER
the flow of current from
to
has been disrupted and their
do not let them to be on
out of phase gate voltages
faces very large impedance
at the same time. As a result,
through rectifier input, and
and
theoretically reach their
and
, respectively. This would
maximum levels,
tank and decreased curresult in increased voltage across
, which can be easily detected esrent in the primary coil
pecially with small
to recover back telemetry data from OC
[see Fig. 7(a)]. In practice, various leakage currents at the rectito be infinitely large as in (9).
fier input do not let
Short Coil: In this mode MUXn
Mode-2
pair connects the gates of
and
to
resulting in
, where
is the nMOS threshold
and
stay on continuously and create
voltage. Therefore,
tank, resulting in
and
a small impedance across
theoretically reach their minimum values,
and
,
respectively. This would result in decreased voltage across
tank and increased current in the primary coil, which
to recover back
can be easily detected especially with large
telemetry data from SC [see Fig. 7(b)]. Practically,
and
on resistances do not let
reach the short circuit level
and
stay off as long as
stated in (8). Diode-connected
and
, eliminating the storage capacitor
which supplies the load in Modes 1 and 2, from being
and .
discharged into
IV. MEASUREMENT RESULTS
A prototype rectifier with built-in back telemetry was dem n-well 3M/2P 5 V
signed and fabricated in the AMI
standard CMOS process. Fig. 5 shows the prototype chip micrograph which includes two rectifiers, enclosed in white boxes,
and a few on-chip ripple rejection capacitors. Each rectifier ocmm of chip area in this process. The measurecupies
ment setup was similar to the block diagram in Fig. 1 [12].
The rectifier was powered by a class-E amplifier operating at
500 kHz through a pair of planar spiral coils, which specifications are summarized in Table I [20].
Fig. 6 shows the rectifier input, body, and output voltage
.
waveforms while operating in Mode-0 with
and
separated n-well regions were not
Even though
physically connected, their body voltages tend to be very
. It can be seen that
follows
close
as a result of
operation and
therefore eliminates latchup and substrate leakage by keeping
all the parasitic components off [19]. Figs. 7(a) and (b) show
the rectifier operation in Modes 1 and 2 when there is a dynamic
and
, respectively. In
load condition with
this experiment, back telemetry data was applied to the OC and
SC inputs as a 15-kHz square wave with 35% duty cycle (first
increases
trace). The second trace in Fig. 7(a) shows how
, which reflects back onto the
as a result of
current .
is measured
primary coil by a reduction in
wire through a current sense transformer, which
by passing
output voltage is shown as
on the fourth trace.
Since the focus of this paper is on the integrated back
telemetry rectifier as part of the transponder, and not the
reader, the current sense transformer was far from optimal
and intended only for showing the primary current variations
3333
on the oscilloscope. In an actual reader, however, the sensing
capability of the current sense transformer can be significantly
improved by enhancing its coupling with . In addition, the
current sense transformer output can be amplified and filtered
around the back telemetry subcarrier spectrum to eliminate the
much larger main carrier interference [1]. This would result in
current variations from an
a sensitive reader that can detect
extended reading range in presence of noise and interference.
deIn contrast with Fig. 7(a), in Fig. 7(b) (Mode-2),
and reflects back onto the
creases as a result of
current. In these experiments,
primary coil by an increase in
we have intentionally chosen a small ripple rejection capacitor,
nF, to demonstrate on the 3rd trace in both figures how
exponentially decays when
or
, during
is temwhich period there is no rectification and the load
porarily supplied by the stored charge in
.
V. CONCLUSION
We have developed an integrated full-wave standard CMOS
rectifier with dual-mode built-in back telemetry for RFID and
implantable microelectronic device applications. The new rectifier eliminates the need for additional large switches for load
modulation, reducing the rectifier dropout voltage and saving
chip area. The new rectifier also provides more flexibility in
choosing the most appropriate LSK-ASK back telemetry mechanism through shorting and/or opening the transponder coil for
any certain application. In complex systems with multiple operating modes and variable dynamic loading requirements, in
order to maximize load variations for back telemetry, OC can be
is small) and SC can be
used when power demand is high (
used when the system is not fully active and has a small power
is large). The ultimate goal in both condiconsumption (
tions is to improve the read range and achieve a more robust
. Alternatively, OC can
back telemetry link by increasing
variations
always be followed by SC to produce maximum
. Further, since Modes 1 and 2 can be distinregardless of
guished on the reader side by a reduction and an increase in the
primary coil current, respectively, it is also possible to assign
more bits to OC and SC combinations to achieve higher back
telemetry data rates.
In order to verify and evaluate the new rectifier topology,
we developed a prototype version of the new rectifier in the
m n-well standard CMOS process, occupying
AMI
mm of chip area. The prototype rectifier was tested and
proved to be fully functional in all three modes of operation:
rectifier, open coil, and short coil.
ACKNOWLEDGMENT
The authors would like to thank the Department of Electrical
and Computer Engineering, North Carolina State University, for
their support and the MOSIS Educational Program for fabricating the prototype chip.
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microstimulating system-on-a-chip with modular architecture,” IEEE
Trans. Neural Syst. Rehab. Eng., vol. 15, no. 3, pp. 449–457, Sep. 2007.
Maysam Ghovanloo (S’00–M’04) was born in
1973 in Tehran, Iran. He received the B.S. degree
in electrical engineering from the University of
Tehran, in 1994 and the M.S. degree in biomedical
engineering from the Amirkabir University of Technology, Tehran, in 1997. He also received the M.S.
and Ph.D. degrees in electrical engineering from the
University of Michigan, Ann Arbor, in 2003 and
2004, respectively. His Ph.D. research was on developing a wireless microsystem for micromachined
neural stimulating microprobes.
In December 1998, he founded Sabz-Negar Rayaneh Co. Ltd., Tehran, to
manufacture physiology and pharmacology research laboratory instruments. In
summer 2002, he was with Advanced Bionics Inc., Sylmar, CA, working on
spinal-cord stimulators. From 2004 to 2007, he was an Assistant Professor with
the Department of Electrical and Computer Engineering, North Carolina State
University, Raleigh, where he founded and directed the NC-Bionics Lab. In June
2007, he joined the faculty of Georgia Institute of Technology, Atlanta, where
he is currently an Assistant Professor in the School of Electrical and Computer
Engineering. He has more than 60 conference and journal publications.
Dr. Ghovanloo is an Associate Editor of the IEEE TRANSACTIONS ON
CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS. He has been a member of the
Technical Program Committee for the IEEE Midwest Circuits and Systems
(MWSCAS), International Symposium on Circuits and Systems (ISCAS),
and Biomedical Circuits and Systems (BioCAS) conferences. He has received
awards in the operational category of the 40th and 41st DAC/ISSCC student
design contest in 2003 and 2004, respectively. He is a member of Tau Beta
Pi, Sigma Xi, and IEEE Solid-State Circuits, Circuits and Systems, and
Engineering in Medicine and Biology societies.
Suresh Atluri (S’05–M’07) was born in India in
1983. He received the B.S. degree in electrical
and electronics engineering from BITS, Pilani,
Rajasthan, in 2004 and the M.S. degree in electrical
engineering from the Department of Electrical
and Computer Engineering, North Carolina State
University, Raleigh, in 2006.
He specialized in analog, digital, and microwave
circuit design. His research was oriented toward developing inductive power and data transfer circuits
for implantable biomedical devices. He joined Integrated Device Technologies Inc., Atlanta, GA, in 2006, where he works as an
electronics design engineer.
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