volume39-number2(1).pdf
(
958 KB
)
Pobierz
A forum for the exchange of circuits, systems, and software for real-world signal processing
Volume 39, Number 2, 2005
In This Issue
Editors’ Notes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Transformer-Coupled Front-End for Wideband A/D Converters
. . . . . . . . . . . . . . . . . . . 3
Pushing the State of the Art with Multichannel A/D Converters
. . . . . . . . . . . . . . . . . . 7
Which ADC Architecture Is Right for Your Application?
. . . . . . . . . . . . . . . . . . . . . . . 11
Product Introductions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
�����������������������������
Editors’ Notes
DATA CONVERTERS
In 1969, the acquisition of Pastoriza
Electronics launched ADI into the world
of analog-to-digital and digital-to-analog
converters. Hot products at the time were
the ADC-F 10-bit, 1-s A/D converter
and the MDA-L 12-bit current-output
D/A converter. Over the succeeding years,
Analog Devices engineers have produced a
series of breakthrough products, including
the monolithic AD7520 10-bit CMOS DAC
in 1974, DAC1138 18-bit modular DAC in
1977, and an endless stream of ICs: AD7541 12-bit multiplying DAC
in 1978, AD574 complete 12-bit ADC in 1980, AD7546 16-bit DAC
in 1981, AD9000 6-bit, 100-MHz ADC in 1984, AD671 12-bit,
2-MHz ADC in 1990, and AD771x 24-bit sigma-delta ADCs in 1992,
to mention but a few of a great many irsts. Today, commanding a
45% share of the worldwide converter market, Analog Devices is the
unquestioned leader in data-conversion technology.
To many engineers, op amps were—and data converters still are—a
mystery, the latter combining the behavioral quirks of both analog
and digital designs. Thus, from the irst issue of
Analog Dialogue
in 1967 and the first printing of the
Analog-Digital Conversion
Handbook
in 1972, Analog Devices has continually been a champion
of education and training, augmenting its state-of-the-art converters
with world-class data sheets, handbooks, magazines, and technical
seminars. In this issue of
Analog Dialogue
, featuring data conversion,
you’ll read about some flagship ADCs, such as the multichip
12-bit, 500-MSPS AD12500 and 16-bit, 80-MSPS AD10678; and the
monolithic 16-bit, 3-MSPS AD7621 and 18-bit, 2-MSPS AD7641.
You’ll also read about ADC architectures, learn how to choose an
A/D converter to it your application, and discover some tricks for
designing a wideband transformer-coupled ADC front-end.
Scott Wayne
[scott.wayne@analog.com]
skills. In addition, an ADI Fellow must be a company ambassador,
bridging across organizations and demonstrating an unparalleled
ability to teach and mentor others within the company. With the latest
inductions, Analog Devices has a total of 30 Fellows out of more than
3,000 engineers worldwide.
“A commitment to engineering excellence is the lifeblood of Analog
Devices, and the talent and dedication that Mike and Katsu bring to
every project they undertake is testimony for that core belief,” said Sam
Fuller, vice president of research & development for ADI. “But what
really singles them out is their constant innovation. It’s this drive that
solves our customers’ problems, generates the revenue that enables ADI
to maintain an aggressive R&D schedule, and sets inspirational goals
for our employees.” Between them, Coln and Nakamura have received
26 patents for inventions created at Analog Devices.
Mike Coln
joined Analog Devices in 1988,
after earning a Ph.D. from MIT. Since then,
he has been involved in design, leadership, and
mentoring roles, contributing to all areas of
precision data converter development within
the company. A holder of 12 patents (with
another four in development), Coln was the
chief architect of ADI’s PulSAR
®
analog-
to-digital-converter (ADC) family, which
overcame perceived architectural barriers
then boxing-in the speciications of speed,
resolution, power consumption, and size of
successive-approximation converters. The PulSAR self-calibrating
architecture was the irst to enable 16-bit ADCs to reach throughput
of 1 MSPS (million samples per second), and it resulted in the irst
SAR ADC to reach 18-bit resolution.
Katsu Nakamura
received his Ph.D. in
Electrical and Computer Engineering
from Carnegie Mellon University in 1994
before joining Analog Devices as a design
engineer. He has been a pivotal force and
the chief architect of technologies leading
ADI to well over 50% share of the market
for
analog front-ends
(AFEs) in digital still
cameras. Nakamura, who holds 14 patents,
guided ADI’s migration of AFE technology
to deep-submicron CMOS processes,
integrating components formerly only
available using bipolar manufacturing techniques.
The fifth generation of these products is now available with
outstanding performance, integrated with complex digital circuitry.
Our AFE customer list has grown to include all major camera
manufacturers—among the most demanding in our industry for
quality, performance, and price.
PulSAR
®
is a registered trademark of Analog Devices, Inc.
PREMIUM PERFORMANCE (HUMAN)
At our 2005 General Technical Conference,
significant awards were given to three
outstanding Analog Devices technologists.
Two new Fellows were named, and—in
celebration of our 40
th
anniversary in
business (1965-2005)—board chairman and
co-founder, Ray Stata, named the recipient
of the irst Analog Devices Founder’s Award.
The details of the awards follow below.
Dan Sheingold
[dan.sheingold@analog.com]
TWO NEW FELLOWS NAMED
Two ADI senior engineers,
Dr. Michael Coln
and
Dr. Katsu Nakamura
,
were named to the distinguished position of ADI Fellow during the
company’s 2005 General Technology Conference (GTC), which
attracted more than 1,500 engineers from the company’s design
sites worldwide.
The Fellows honor is awarded when an engineer has contributed
signiicantly to ADI’s business and demonstrated important qualities,
such as innovation, leadership, entrepreneurial ability, and consulting
WINNER OF FIRST FOUNDER’S
INNOVATION AWARD
In honor of our 40
th
anniversary,
Analog Devices has established
a new award, the
Founder’s
Innovation Award
.
Steve Sherman
was named as the irst recipient of
this award, at the 2005 GTC, by
Ray Stata, ADI’s co-founder (in
1965) and board chairman.
Every day, somewhere in the
world, an airbag reliably deploys during a car crash and saves a life—
thanks to the vision and determination of a certain Analog Devices
employee. To honor the engineer who championed the development
of
i
MEMS
®
(
integrated microelectromechanical systems
) technology and
helped pioneer its application to automobile airbags, ADI awarded
the irst Founder’s Innovation Award to Steve Sherman.
Announcing the award, Ray said: “It’s really rare that the imagination,
dedication, persistence, and innovative skills of a single individual
would create an entirely new business—now with proitable sales
of over $100 million, would have established ADI as the largest
manufacturer of MEMS devices in the world, and would have gained
for this company the recognition as a leader in that technology. It
simply would not have happened at ADI if Steve did not passionately
embrace and champion a vision of opportunity in an area outside of
continued on page 18
www.analog.com/analogdialogue
dialogue.editor@analog.com
Analog Dialogue
is the free technical magazine of Analog Devices, Inc., published
continuously for 39 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].
ISSN 0161-3626 ©Analog Devices, Inc. 2005
Transformer-Coupled Front-End
for Wideband A/D Converters
C2
C3
1:1
Z RATIO
R1
L1
L3
R3
1
3
By Rob Reeder [
rob.reeder@analog.com
]
R
CORE
C1
C6
INTRODUCTION
With the push into higher-frequency IF sampling, the analog
inputs and overall front-end design of the A/D converter have
become crucial elements of receiver design. Many applications are
migrating to super-Nyquist sampling in order to eliminate a mix-
down stage in the system design. Ampliiers pose a problem at these
high frequencies, because high performance isn’t as easy to achieve
as in the Nyquist applications for which they are typically used.
In addition, the ampliier’s inherent noise will degrade the ADC’s
signal-to-noise ratio (SNR), no matter what input frequency is
used. A transformer provides the designer with a relatively easy
solution that resolves the noise issue, while providing a good
coupling mechanism for high-frequency inputs.
PRIMARY
SECONDARY
R2
R4
2
4
L2
L4
C4
C5
Figure 1b. Typical transformer model.
Figure 1b shows many of the inherent and parasitic departures
from the ideal that come into play with a transformer. Each of these
has a role in establishing the transformer’s frequency response.
They can help or hinder performance, depending on the front-
end implementation. Figure 1b provides a good way to model a
transformer to get irst-order expectations. Some manufacturers
provide modeling information, either on their website or through
a support group. Anyone planning to do the model analysis using
the hardware will need a network analyzer and a handful of samples
to make all of the measurements properly.
Real transformers have losses and limited bandwidth. As the
coniguration of parasitics implies, one can think of a transformer
as a wideband band-pass ilter, which can be deined in terms of
its –3-dB points. Most manufacturers will specify transformer
frequency response in terms of the 1-, 2-, and 3-dB bandwidth.
The amplitude response is accompanied by a phase characteristic.
Usually a good transformer will have a 1%-to-2% phase imbalance
over its frequency pass band.
Let us now consider some design examples involving a transformer-
coupled front-end for an ADC. Since the transformer is used
primarily for isolation and center-tapping, these examples will be
simpliied for discussion by using a unity turns ratio.
The Transformer
Let us look at the basic makeup of a transformer and summarize
what it provides to the user. First, the transformer is inherently
ac-coupled, since it is galvanically isolated and will not pass dc
levels. It provides the designer with basically noise-free gain, which
depends on the designer’s choice of turns ratio. The transformer
also provides a quick and easy way of translating from a single-
ended to a differential circuit. Finally, a center-tapped transformer
provides the freedom to set the common-mode level arbitrarily.
This combination of virtues reduces component count in front-end
designs, where it is critical to keep complexity at a minimum.
However, care should be taken when using center-tapped
transformers. If the converter circuit presents large imbalances
between the differential analog inputs, a large amount of current
could flow through the transformer’s center tap, possibly
saturating the core. For example, instability could result if V
REF
is used to drive the center tap of the transformer, and a full-
scale analog signal overdrives the ADC’s input, turning on the
protection diodes.
Although simple in appearance, transformers should not be taken
lightly. There is much to know about and learn from them. Let’s
look at a simple model of the transformer and see what is “under
the hood.” A couple of simple equations relate the currents and
voltages occurring at the terminals of an ideal transformer, as
shown in Figure 1. When voltage is stepped up by a transformer,
its impedance load will be relected back to the input. The turns
ratio,
a = N
1
/N
2, deines the ratio of primary voltage to secondary
voltage; the currents are inversely related (
a = I
2
/I
1), and the
ratio of the impedance seen in the primary relected from the
secondary goes as the square of the turns ratio (
Z
1
/Z
2
= a
2
). The
transform
er’s sign
al gain is expressed simply as 20 log (V2/V1)
= 20 log ÷
(
Z2/Z1
)
, so a transformer with a voltage gain of 3 dB
would have a 1:2 impedance ratio. That makes for an easy irst
step of the design.
Examples
In the irst example, shown in Figure 2, an
AD6645
1
14-bit,
80-MSPS ADC, with a differential input impedance of 1 kohm,
is used. The 33-ohm series resistors provide isolation from
transient currents in the input circuit of the ADC. The 501-ohm
terminating resistor is chosen to achieve a 50-ohm input on the
primary to match the 50-ohm analog input source. Thus
(
+
(
)
)
=
(1)
R
in
=
58 66 501 1000
Ω Ω
Ω
Ω
5065
.
Ω
The resistive combination in the transformer secondary is
effectively in parallel with the 58-ohm resistor. The choice of
terminating resistor depends on the desired input impedance. For
simplicity, it will be assumed that a match to a 50-ohm source is
required for all of the examples in this section.
XFMR
1:1 Z
33
AIN+
ANALOG
INPUT
INPUT
Z = 50
AD6645-80
I1
I2
58
2pF
501
1000
1.5pF
1
3
ADC
INTERNAL
INPUT Z
AIN–
33
PRIMARY
SECONDARY
V1 (Z1)
V2 (Z2)
0.1
F
2
4
1:N TURNS
Figure 2. A 1:1 transformer coupling a 50-ohm input
source with an ADC having a known input impedance.
Figure 1a. Transformer input and output variables.
Analog Dialogue Volume 39 Number 2 3
This is an easy example because we assume that the input
frequency is in baseband or irst Nyquist zone. However, the
situation is quite different if the front-end design is called
on to handle a 100-MHz analog input. What happens in the
transformer? With such a high IF frequency applied, any
difference in parasitic capacitive coupling (C2–C5 in
Figure 1b) unbalances the secondary outputs of the transformer.
The resulting asymmetry gives rise to even-order distortions at
the converter’s analog input, which leads to 2
nd
-order harmonic
distortions in the digital signal.
To illustrate this point, Figure 3 shows the voltages on the
secondary when a 2-V p-p sinusoidal input is applied to the
primary (100 MHz in Figure 3a and 200 MHz in Figure 3b).
The secondary outputs are each expected to produce a 1-V p-p
sine wave. But at 100 MHz, their amplitudes deviate by
10.5 mV p-p, with 0.5 phase imbalance. And at 200 MHz,
the amplitude difference is 38 mV p-p, or 1.9%.
One way to improve the situation is to apply a second transformer
in cascade with the irst to provide additional isolation and reduce
the unbalanced capacitive feedthrough (Figure 4).
XFMR
1:1 Z
XFMR
1:1 Z
33
AIN+
ANALOG
INPUT
INPUT
Z = 50
AD6645-80
OPTIONAL
58
501
1000
1.5pF
ADC
INTERNAL
INPUT Z
AIN–
33
0.1
F
0.1
F
Figure 4. Cascaded transformers.
Using this scheme, the differential voltages applied to
the converter are less likely to deviate from one another,
particularly at high frequencies where this matters most.
Figure 5 illustrates this point: the first transformer’s secondary
differences in parasitic coupling capacitances, C1 and C2,
are reduced. The second transformer in cascade enables a
redistribution of the core current lost and provides more equal
signals to the primary of the second transformer. The two
cascaded transformers in this configuration provide a better
balanced solution for high frequencies.
XFMR
1:1 Z
V(AIN+)
V(AIN–)
1.5
(1.0725
s, +681.963mV) (1.0879
s, +677.224mV)
1.0
XFMR
1:1 Z
0.5
2V p-p
1V p-p
1V p-p
C
1
ANALOG
INPUT
0
I
CORE
–0.5
C
2
0.9V p-p
0.95V p-p
–1.0
(1.0775
s, –682.450mV) (1.0929
s, –676.740mV)
Figure 5. Two transformers in cascade improve
signal balance.
–1.5
The performance benefit can be seen in Figure 6 from the
simulation. In Figure 6a, with an analog input of 100 MHz, the
deviation drops to 0.25 mV p-p, or 0.013%. And at 200 MHz
(Figure 6b), there is only a 0.88 mV p-p difference between
the transformer’s secondary outputs, or 0.044%. This is a big
improvement, attained by adding one extra component.
1.060 1.065 1.070 1.075 1.080 1.085 1.090 1.095 1.100
TIME (
s)
Figure 3a. 100-MHz input. Simulation of the transformer’s
secondary outputs: AIN+ (green) = 1.364 V p-p, AIN–
(red) = 1.354 V p-p, Difference = 10.45 mV p-p.
V(AIN+)
V(AIN–)
V(AIN+)
V(AIN–)
1.5
800
(3.0125
s, +625.226mV)
(3.0275
s, +625.154mV)
(1.0663
s, +692.384mV) (1.0791
s, +673.768mV)
1.0
400
0.5
0
0
–0.5
–400
(1.0688
s, –692.628mV)
(1.0816
s, –673.526mV)
–1.0
(3.0175
s, –625.427mV) (3.0325
s, –625.247mV)
–1.5
–800
1.060 1.065 1.070 1.075 1.080 1.085 1.090 1.095 1.100
3.000 3.005 3.010 3.015 3.020 3.025 3.030 3.035 3.040
TIME (
s)
TIME (
s)
Figure 3b. 200-MHz input. Simulation of the transformer’s
secondary outputs: AIN+ (green) = 1.385 V p-p, AIN–
(red) = 1.347 V p-p, Difference = 37.72 mV p-p.
Figure 6a. 100 MHz. Simulation of the transformer’s
secondary outputs: AIN+ (green) = 1.25 V p-p, AIN–
(red) = 1.25 V p-p, Difference = 0.25 mV p-p.
4 Analog Dialogue Volume 39 Number 2
V(AIN+)
V(AIN–)
0
1.0
(3.0063
s, +647.702mV) (3.0189
s, +650.243mV)
–0.5
–1.0
0.5
–1.5
–2.0
–2.5
0
–3.0
–3.5
–0.5
–4.0
–4.5
(3.0089
s, –651.281mV) (3.0213
s, –647.862mV)
–1.0
–5.0
3.000 3.005 3.010 3.015 3.020 3.025 3.030 3.035 3.040
1
10
100
1000
TIME (
s)
FREQUENCY (MHz)
Figure 6b. 200 MHz. Simulation of the transformer’s
secondary outputs: AIN+ (green) = 1.298 V p-p, AIN–
(red) = 1.298 V p-p, Difference = 0.88 mV p-p.
Figure 8a. Frequency response of a typical transformer.
1
0nH
Another way to approach this is to use a two-balun type
transformer coniguration. A balun (balance-unbalance) acts
like a transmission line and usually has greater bandwidth than
the standard lux type transformers discussed earlier. They can
provide good isolation between the primary and secondary with
relatively low loss. However, they require more power to drive
because the input impedance is halved from the primary to the
secondary. Figure 7a shows a common implementation that is used
in order to achieve a wide pass band. In Figure 7b, the balun type
transformer is precompensated for the imbalance.
51nH
100nH
0
150nH
200nH
250nH
330nH
–1
390nH
–2
–3
Response Peaking
Figure 8a shows a typical transformer frequency response,
essentially that of a wideband ilter with bandwidth in excess of
100 MHz. An inductor in series with the transformer’s primary
can be used to alter the bandwidth response of the transformer, by
peaking the gain in the pass band and providing a steeper roll-off
outside the pass band (Figure 8b). The inductor has the effect of
adding a zero and a pole in the transfer function.
–4
–5
0
50
100
150
200
250
300
350
400
450
500
FREQUENCY (MHz)
Figure 8b. Frequency response of a typical transformer
with an inductor in series.
0.1
F
33
AIN+
ANALOG
INPUT
INPUT
Z = 50
AD6645-80
BALUN
1:1 Z
58
501
1000
1.5pF
OPTIONAL
ADC
INTERNAL
INPUT Z
AIN–
33
0.1
F
BALUN
1:1 Z
Figure 7a. Transformer-coupled input using a two-balun type transformer coniguration.
0.1
F
33
ANALOG
INPUT
INPUT
Z = 50
AIN+
AD6645-80
BALUN
1:1 Z
58
501
1000
1.5pF
OPTIONAL
ADC
INTERNAL
INPUT Z
33
AIN–
0.1
F
Figure 7b. Transformer-coupled input using a compensated-balun type transformer.
Analog Dialogue Volume 39 Number 2 5
Plik z chomika:
TirNaNog
Inne pliki z tego folderu:
volume1-number3(1).pdf
(7733 KB)
volume1-number2(1).pdf
(3887 KB)
volume10-number2(1).pdf
(3533 KB)
volume10-number1(1).pdf
(3227 KB)
volume1-number1(1).pdf
(1913 KB)
Inne foldery tego chomika:
ACE
AcornUser
AmigaComputing
AmigaFormat
AmigaShopper
Zgłoś jeśli
naruszono regulamin