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Volume 38, Number 4, 2004
In This Issue
In This Issue
Editors’ Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Wideband CMOS Switches (Ask The Application Engineer—34) . . . . . . . . . . . . . . . . . . . .
3
Support for the Designer—Improved ADI Website Helps You . . . . . . . . . . . . . . . . . . . . . . .
8
Designing Efficient, Real-Time Audio Systems with VisualAudio™ . . . . . . . . . . . . . . . . .
11
Recent Product Introductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
Editors’ Notes
You’ve opened the book on the final
quarterly issue of 2004, our 38
th
sequential year in print—and 6
th
online,
at
analog.com/analogdialogue
. Perhaps
you’ve read all four issues cover to cover.
Or perhaps this is your irst acquaintance
with
Analog Dialogue
. In any event, here’s
your opportunity to spend a moment to be
tempted to read an article you may have
missed—or to contemplate a title that
you’ve already read. You can ind copies of all these issues online
in the archives at http://www.analog.com/library/analogdialogue/
archives.html
The year started—in Number 1—with PID control algorithms,
fan-speed in temperature control, and video technology in
automotive safety. In this column, you could have read a
rambling historical discourse on (mostly—but not entirely—
analog) multipliers.
In the following issue—Number 2—you (could have) read about
current measurement in solenoids for automotive controls, bridge
ampliication with digitally programmed gain and offset, and
practical techniques to avoid op-amp instability due to capacitive
loading. There was also a description of techniques we use for
in-package trimming of a low-cost CMOS ampliier with wide
bandwidth, offsets less than 65 V and drifts less than 7 V/C.
The penultimate issue—Number 3—had an “Ask The
Applications Engineer” (#33) feature on
direct digital synthesis
(DDS), plus articles on JPEG2000 image compression and
a digitally adjustable cable equalizer. You also could have
read about a reader’s discovery—in a NASA vehicle, in
equipment designed before he was born—of an ingenious but
deceptively simple hot-wire anemometer. Its design principle
was described in a (still interesting) article on measuring
fluid flow with a self-balancing bridge; originally appearing
in our Volume 5, in 1971; it was reprinted in this issue.
And in these pages today you can read about our designer-oriented
updated website, a new software tool for memory-eficient, real-
time audio designs, and an “Ask The Applications Engineer” (#34)
on wideband CMOS switches.
Thus we close the book on Volume 38 and look forward eagerly to
Volume 39, which will commemorate Analog Devices’s 40th year
of providing the electronic industry with innovative products,
guidance, and ideas for analog- and digital real-world signal
processing solutions.
SATELLITE RADIO, MP3s, AND STREAMING AUDIO
Back in the days of analog LPs on vinyl,
I owned over 500 record albums. Then,
as an early adopter of compact discs, I
bought all of my new music on CDs,
and even started to replace some of my
records. Soon I abandoned the turntable
altogether and gave all of my albums to
my brother. Sadly, a flood ruined all the
records, but I shed nary a tear, exulting
in the luxury of the newer, smaller,
virtually indestructible CDs.
Yet lately I’ve realized that I rarely buy CDs anymore—and
when I listen to them it’s almost always in the car. In my family
room I usually listen to one of the dozens of commercial-free,
CD-quality audio channels that are available over the digital
cable. It makes available a much wider variety of music, and
lets me view trivia, history, and other information on the song,
album, and artist. When at the computer, I listen to streaming
audio from one of several providers, using one of the available
media players. The small annual payment for this service
makes it possible to listen to high-quality audio from over
1,000 stations and lets me download my favorite songs for a
nominal additional fee.
At the gym, at the beach, or in the backyard, I listen to my MP3
player. Where do the MP3s come from? Most were ripped from
my CD collection, but the newest ones are all downloads. Why
buy the whole CD when I need buy only my favorite songs—for
a fraction of the cost—and eliminate the storage problem at
the same time.
In the car, I listen mostly to the radio, but am constantly
annoyed and frustrated by the large number of commercials,
especially at drive time. Although I listen to CDs in the car,
those jewel boxes take up too much space and are too hard
to open safely while driving. CDs in sleeves are more space
efficient and are easier to handle, but they’re sometimes hard
to identify without their covers. An FM modulator lets me
listen to my MP3 player in the car, but the audio quality is not
as good as a CD, and it’s sometimes difficult to find an unused
radio frequency in Boston’s busy metropolitan market.
Thus, one option that now tops the priority list for my new car
is satellite radio, either XM or Sirius. In the early days of cable,
skeptics wondered why people would pay to watch TV when
they could watch it for free. Today, many people feel the same
way about radio, but I look forward to the day when I can give
my CD collection to my brother and rely on streaming media
wherever I go.
Dan Sheingold
[dan.sheingold@analog.com]
www.analog.com/analogdialogue
dialogue.editor@analog.com
Analog Dialogue
is the free technical magazine of Analog Devices, Inc., published
continuously for 38 years—starting in 1967. It discusses products, applications,
technology, and techniques for analog, digital, and mixed-signal processing. It is
currently published in two editions—
online
, monthly at the above URL, and quarterly
in print
, as periodic retrospective collections of articles that have appeared online. In
addition to technical articles, the online edition has timely announcements, linking to
data sheets of newly released and pre-release products, and “Potpourri”—a universe
of links to important and rapidly proliferating sources of relevant information and
activity on the Analog Devices website and elsewhere. The
Analog Dialogue
site is,
in effect, a “high-pass-iltered” point of entry to the
www.analog.com
site—the
virtual world of
Analog Devices
. In addition to all its current information, the
Analog Dialogue
site has archives with all recent editions, starting from Volume 29,
Number 2 (1995), plus three special anniversary issues, containing useful articles
extracted from earlier editions, going all the way back to Volume 1, Number 1.
If you wish to subscribe to—or receive copies of—the print edition, please go to
www.analog.com/analogdialogue
and click on
<subscribe>
. Your comments
are always welcome; please send messages to
dialogue.editor@analog.com
or to these individuals:
Dan Sheingold
, Editor
[dan.sheingold@analog.com]
or
Scott Wayne
, Managing Editor and Publisher
[scott.wayne@analog.com]
.
Why am I writing about this here? Because as I drive to work
each day I can feel proud that Analog Devices offers ampliiers,
converters, and processors that enable satellite receivers, set-top
boxes, computer audio, and MP3 players to be small, lexible,
power-efficient, and inexpensive—all the while providing
high quality and functionality—plus the software that helps
developers to quickly bring these products to market.
Your comments are welcome.
Scott Wayne
[scott.wayne@analog.com]
ISSN 0161-3626 ©Analog Devices, Inc. 2005
Ask The Application Engineer—34
Wideband CMOS Switches
By Theresa Corrigan [theresa.corrigan@analog.com]
Q:
You mention off isolation and insertion loss. Could you explain what
these are?
A:
Yes, the two most important parameters that describe the
performance of an RF switch are the
insertion loss
in the closed
state and the
isolation
in the open state.
Off isolation
is deined as the attenuation between input and
output ports of the switch when the switch is
off
.
Crosstalk
is a
measure of the isolation from channel to channel.
For example, the ADG919 SPDT switch provides about
37 dB of isolation at 1 GHz, as shown in Figure 2. The
same device, using the chip-scale package (CSP)—offered
for space-constrained wireless applications, such as antenna
switching—offers a 6-dB improvement (43 dB at 1 GHz).
Q:
What is a CMOS wideband switch?
A:
CMOS wideband switches are designed primarily to meet the
requirements of devices transmitting at ISM (
industrial, scientiic,
and medical
) band frequencies (900 MHz and up). The low
insertion loss, high isolation between ports, low distortion, and
low current consumption of these devices make them an excellent
solution for many high frequency applications that require low
power consumption and the ability to handle transmitted power
up to 16 dBm. Examples of applications mentioned later in this
article, include car radios, antenna switching, wireless metering,
high speed iltering and data routing, home networking, power
ampliiers, and PLL switching.
Q:
How do these switches come to be so much faster than typical analog
CMOS switches?
A:
To improve their bandwidth, wideband switches use only
N-channel MOSFETs in the signal path. An NMOS-only
switch has a typical –3-dB bandwidth of 400 MHz—almost
twice the bandwidth performance of a standard switch with
NMOS and PMOS FETs in parallel. This is a result of the
smaller switch size and greatly reduced parasitic capacitance
due to removal of the P-channel MOSFET. N-channel
MOSFETs act essentially as voltage-controlled resistors.
The switches operate as follows:
V
gs
>
V
t
Æ Switch ON
V
gs
<
V
t
Æ Switch OFF
Where V
gs
is the gate-to-source voltage and V
t
is deined as the
threshold voltage
—above which a conducting channel is formed
between the source and drain terminals.
As the signal frequency increases to greater than several
hundred megahertz, parasitic capacitances tend to dominate.
Therefore, achieving high
isolation
in the switches’
off
-state
and low
insertion loss
in the
on
state for wideband applications is
quite a challenge for switch designers. The channel resistance
of a switch must be limited to less than about 6 ohms to achieve
a low-frequency insertion loss of less than 0.5 dB on a line
with 50-ohm matched impedances at the source and load.
As a departure from the familiar switch topology, inserting
a shunt path to ground for the
off
-throw—and its associated
stray signal—allows the design of switches with increased off-
isolation at high frequencies. The FETs have an
interlocking
inger
layout that reduces the parasitic capacitance between
the input (RFx) and the output (RFC), thereby increasing
isolation at high frequencies and enhancing crosstalk rejection.
For example, when MN1 is
on
to form the conducting path
for RF1, MN2 is
off
and MN4 is
on
, shunting the parasitics at
RF2 to ground, as shown in Figure 1.
0
V
DD
= 1.65V TO 2.75V
T
A
= 25
C
–10
–20
–30
–40
–50
S12
–60
–70
–80
S21
–90
–100
10k
100k
1M
10M
100M
1G
10G
FREQUENCY (Hz)
Figure 2. Off isolation vs. frequency.
Insertion loss
is the attenuation between input and output ports of
the switch when the switch is
on
. The switch is generally one of
the irst components encountered in a receiver’s signal path, so a
low insertion loss is required to ensure minimum signal loss. Low
switch insertion loss is also important for systems that require a
low overall noise igure.
To obtain the best insertion-loss performance from the ADG9xx
family of switches, one should operate the part at the maximum
allowable supply voltage of 2.75 V. The reason can be seen in
Figure 3, which shows plots of insertion loss versus frequency for
the ADG919 at three different values of supply voltage.
–0.30
–0.35
V
DD
= 2.75V
V
DD
= 2.5V
–0.40
–0.45
–0.50
V
DD
= 2.25V
–0.55
RF COMMON
–0.60
MN1
MN2
–0.65
RF1
RF2
–0.70
R1
R2
R3
–0.75
T
A
= 25
C
MN3
R4
MN4
–0.80
10k
100k
1M
10M
100M
1G
10G
FREQUENCY (Hz)
Figure 3. Insertion loss vs. frequency.
IN
Figure 1. A typical transistor based Tx/Rx switch.
Analog Dialogue Volume 38 Number 4 3
Q:
How does insertion loss relate to the On-resistance spec of a standard
analog switch?
A:
Signal loss is essentially determined by the attenuation
introduced by switch resistance in the
on
condition,
R
on
, in
series with the source-plus-load resistance—measured at
the lower frequencies of operation. Figure 4 shows a typical
proile of
on
-resistance as a function of source voltage for an
N-channel MOSFET device.
28
low insertion loss (0.5 dB) all the way down to dc. In addition to
providing a smaller, more eficient design solution, the ADG9xx
family is less power-demanding, consuming less than 1 A over
all voltage and temperature conditions.
Q:
How about the ESD
(electrostatic discharge)
performance as
compared to GaAs?
A:
The ADG9xx family of parts passes the 1-kV ESD HBM
(
human body model
) requirement. ESD protection circuitry is
easily integrated on these CMOS devices to protect the RF and
digital pins. This makes the switches ideal for any applications
that are ESD sensitive, and they offer a reliable alternative to
GaAs devices having ESD ratings as low as 200 V.
Q:
What are the other important speciications of these switches?
A:
Video Feedthrough
(Figure 5) is the spurious dc transient
present at the RF ports of the switch when the control voltage
is switched from high- to- low-, or low- to- high, without an
RF signal present. This is analogous to
charge injection
of a
typical analog switch. It is measured in a 50-ohm test setup,
with 1-ns (rise-time) pulses and a 500-MHz bandwidth.
24
20
16
12
8
T
4
0
0.4
0.8
1.2
1.6
2.0
2.4
1
V
S
(V)
Figure 4. On resistance vs. source voltage.
Q:
What technologies have been commonly used in the design of high-
frequency switches?
A:
Traditionally, only a few processes were available for developing
good wideband/RF switches. Gallium arsenide (GaAs) FETs,
PIN diodes, and electromechanical relays have dominated the
market, but standard CMOS is now a strong entry.
PIN diodes are highly linear devices with good distortion
characteristics, but they have many drawbacks given today’s
high performance demands. They have very slow switching
times (microseconds, compared to nanoseconds for CMOS
switches); they are power-hungry, making them unsuitable for
many battery-operated devices; and—unlike CMOS switches
with their response from RF to dc—there is a practical lower
frequency limit to the use of PIN diodes as linear switches.
GaAs has been popular because of its low
on
resistance, low
off
capacitance, and high linearity at high frequencies. As
CMOS process geometries continue to shrink, however, the
performance of CMOS switches has increased to the extent
that they can achieve –3-dB frequencies of up to 4 GHz and
are able to compete with GaAs switches. Designed to maximize
bandwidth while maintaining high linearity and low power
consumption, CMOS switches now offer a practical alternative
to GaAs switches in many low-power applications.
Q:
So what are the main beneits of CMOS wideband switch solutions
over gallium arsenide?
A:
Switches, such as the ADG9xx family of parts, have an
integrated TTL driver that allows easy interfacing with other
CMOS devices, since CMOS is compatible with LVTTL logic
levels. The small size of devices with integrated drivers is a
solution for many space-constrained applications.
GaAs switches, as such, need dc-blocking capacitors in series with
the RF ports, effectively loating the die relative to dc ground, so
that the switches can be controlled with positive control voltages.
Wideband switches, such as the ADG9xx family, do not have
this requirement, eliminating concerns of reduced bandwidth,
the impact of the capacitors on overall system performance, and
the extra space and cost of GaAs solutions. Eliminating the
blocking capacitors allows the ADG9xx parts to maintain their
CTRL
RFC
2
CH2 p-p
2.002mV
CH1 500mV
CH2 1mV
M10.0ns
Figure 5. Video feedthrough.
P1dB (1-dB compression point)
is the RF input power level at
which the switch insertion loss increases by 1 dB over its low-level
value. It is a measure of the RF power-handling capability of the
switch. As shown in Figure 6, the ADG918 has a P1dB of 17 dBm
at 1 GHz, with V
DD
= 2.5 V.
Q:
What does this mean?
A:
It means that if the
insertion loss
at 1 GHz was 0.8 dB with a
low-level input, it would be 1.8 dB with a 17-dBm input signal
[Note: dBm is the dB (logarithmic) measure of the ratio of
power to 1 mW, or voltage to 224 mV in 50 ohms. 17 dBm
corresponds to 50 mW, or 1.6 V rms or 4.5 V p-p].
20
18
16
14
12
10
8
6
4
V
DD
= 2.5V
T
A
= 25
C
2
0
0
250
500
750
1000
1250
1500
FREQUENCY (MHz)
Figure 6. 1-dB compression point vs. frequency.
4 Analog Dialogue Volume 38 Number 4
Q:
Power-handling capability seems to decrease substantially at the
lowest frequencies in Figure 6. Why?
A:
In
normal
operation, the switches can handle a 7-dBm (5-mW)
input signal. For a 50-ohm load, this corresponds to a
0.5-V rms signal, or 1.4 V peak-to-peak for sine waves.
[V p-p = V rms 2 ÷
2
].
The power-handling capability is reduced at lower frequencies
for two reasons:
Both of the above mechanisms can be overcome by applying a small
dc bias (about 0.5 V) to the RF input signal when the switch is
being used at low frequencies (<30 MHz) and high power—greater
than 7 dBm (or 5 mW, 1.4 V p-p in 50 ohms). This will raise the
minimum level of the sine-wave input signal and thus ensure
that the parasitic diodes are continually reverse-biased and that
the shunt transistor, never seeing V
gs
> V
t
, remains in the
off
state
for the whole period of the input signal. Figure 9 again shows a
plot of input- and output signals at 100 MHz and 10 dBm input
power (about 2 V p-p in 50 ohms), but this time with a 0.5-V dc
bias. It is clearly visible that clipping or compression no longer
occurs at 100 MHz.
V
G
V
S
V
D
50
50
N+
N+
P TYPE
SUBSTRATE
REF1 FREQ
99.98MHz
REF1 AMPL
1.85V
Figure 7. Physical NMOS structure.
T
As shown in Figure 7, the inherent NMOS structure consists of two
regions of N-type material in a P-type substrate. Parasitic diodes
are thus formed between the N and P regions. When an ac signal,
biased at 0 V dc, is applied to the
source
of the transistor, and V
gs
is large enough to turn the transistor on (V
gs
> V
t
), the parasitic
diodes can be forward-biased for some portion of the negative half-
cycle of the input waveform. This happens if the input sine wave
goes below approximately –0.6 V, and the diode begins to turn
on, thereby causing the input signal to be clipped (compressed),
as shown in Figure 8. The plot shows a 100-MHz, 10-dBm input
signal and the corresponding 100-MHz output signal. It is readily
seen that the output signal has been truncated.
C1 FREQ
100.00MHz
C1 AMPL
1.75V
CH1
500mV
M2.00ns
CH1 0V
Figure 9. 100-MHz, 10-dBm input/output signals
with 0.5-V dc bias.
Q:
How do I apply a dc bias to RF inputs?
A:
To minimize any current drain through the termination
resistance on the input side, it is best to add the bias on the
output (RFC) side. This is the best practice, especially for
low-power portable applications, but it may be necessary to
apply dc-blocking capacitors on the RF outputs if downstream
circuitry cannot handle the dc bias.
Q:
Can these switches operate with a negative supply?
A:
They can operate with a negative signal on the GND (
ground
)
pin as long as it adheres to the –0.5 V to +4 V Absolute
Maximum Rating for V
DD
to GND. Note that operating the
part in this manner places the internal terminations at this new
GND potential—an undesirable effect in some applications.
Q:
What about the distortion performance of these switches?
A:
When tones at closely spaced frequencies are passed through
a switch, the nonlinearity of the switch causes false tones to be
generated, causing undesired outputs at other frequencies. In
communications systems, where channels are becoming more
tightly spaced, it is essential to minimize this
intermodulation
distortion
(IMD) to ensure minimum interference. Applying
two closely spaced equal-power signals with a set frequency
spacing (e.g., 900 MHz and 901 MHz) to the input of a device
under test (DUT), results in the output spectrum shown in
Figure 10. The 3
rd
-order harmonic, usually expressed in dBc,
is the log of the ratio of the power in the 3rd order harmonic
to the power of the fundamental. The larger the (negative)
value, the lower the distortion. Sending these tones through
the ADG918, using a combiner with an input power of 4 dBm,
resulted in an IP3 of 35 dBm as shown in Figure 11. [
Note
: an
excellent discussion of various types of distortion can be found
in “Ask The Applications Engineer—13”]
1
REF1 FREQ
99.98MHz
REF1 AMPL
1.85V
T
C1 FREQ
100.05MHz
C1 AMPL
1.51V
CH1
500mV
CH1 0V
Figure 8. 100-MHz, 10-dBm input/output signals
with 0-V dc bias.
M2.00ns
At low frequencies, the input signal is below the –0.6 V level for
longer periods of time, and this has a greater impact on the 1-dB
compression point (P1dB).
The second reason why parts can handle less power at lower
frequencies is the partial turn-on of the shunt NMOS device
when it is supposed to be
off
. This is very similar to the mechanism
described above where there was partial turn-on of the parasitic
diode. In this case, the NMOS transistor is in the
off
state, with
V
gs
< V
t
. With an ac signal on the source of the shunt device,
there will be a time in the negative half-cycle of the waveform
where V
gs
> V
t
, thereby partially turning on the shunt device.
This will compress the input waveform by shunting some of its
energy to ground.
Analog Dialogue Volume 38 Number 4 5
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