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Linear Technology Magazine V18N1 - March 2008
LINEAR TECHNOLOGY
MARCH 2008
VOLUME XVIII NUMBER 1
IN THIS ISSUE…
Complete IF Receiver
Has 16-Bit, 130Msps ADC,
Fixed-Gain Amplifier
and Antialias Filter in
11.25mm × 11.25mm
µModule Packag by Todd Nelson
Introduction
In the design of high speed receiv-
ers for communications, test or
instrumentation equipment, several
specialized disciplines converge in
one place—the analog-to-digital con-
verter (ADC). Unfortunately, the ADC
is not a simple black box where an
RF designer applies the signal and a
digital designer retrieves the accurate
output. Careful design of the signal
conditioning circuitry to drive the ADC
is critical. Something as seemingly
straightforward as board layout can
degrade the downstream signal by a
few precious decibels. The problem
is that the disciplines required for
the engineering on either side of the
ADC, namely RF/IF design and digital
design, do not include mastery of the
art of ADC interface design. Someone
has to put in the effort to properly drive
the ADC. But who? Instead of adding
more work to either designer’s plate,
what if the ADC were really a black
box, already loaded with integrated
signal conditioning components in an
optimized layout? Now, that would be
a better solution.
The LTM9001 is built using
Linear Technology’s µModule
technology to create an IC
form factor System-in-a-
Package (SiP) that includes
a high speed 16-bit ADC,
antialiasing filter and a low
noise, differential amplifier
with fixed gain. It can
digitize wide dynamic range
signals with an intermediate
frequency (IF) range
up to 300MHz.
The LTM9001 is exactly that black
box. It is built using Linear Technol-
ogy’s µModule ™ technology to create
an IC form factor System-in-a-Package
(SiP) that includes a high speed 16-
bit ADC, antialiasing ilter and a low
noise, differential ampliier with ixed
gain. It can digitize wide dynamic range
DESIGN IDEAS
....................................................31–36
(complete list on page 31)
continued on page
L , LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView,
µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ , No Latency Delta-Sigma, No R SENSE , Operational
Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, True Color PWM,
UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the
companies that manufacture the products.
LINEAR TECHNOLOGY
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L LINEAR IN THE NEWS
Linear in the News…
New µModule Receiver Family Launched
Recently, Linear introduced a new family of signal chain
µModule receiver products. The LTM9001, the irst in a
series of System in Package (SiP) signal chain receiver
modules, uses Linear’s breakthrough µModule packaging
technology, now incorporated in a growing family of power
µModule DC/DC controllers. The LTM9001 is a semi-cus-
tomizable IF/baseband receiver subsystem that includes
a high performance 16-bit Analog-to-Digital converter
(ADC) sampling
up to 160Msps,
an antialiasing
ilter, and ixed
gain differen-
tial ADC driver.
The LTM9001
µModule receiv-
er is applicable
in high sensi-
tivity wireless
b a s e s t a t i o n s
and high reso-
lution instrumentation. Systems designers beneit from
simpliied design and test, consistent high performance,
a compact footprint and the elimination of layout-related
performance problems.
Announced in early February, the LTM9001 µModule
receiver has already received coverage in several major
technical publications. The product will be featured in
upcoming cover articles in High Frequency Electronics in
the US, Electronic Product Design in the UK, Elektronik
Informationen in Germany, as well as in articles in key
publications throughout Asia.
The LTM9001 is gaining signiicant interest from receiver
manufacturers as a way reduce time-to-market while
improving functionality. It does so by delivering a high
level of integration without compromising performance.
The device combines RF, digital and mixed-signal tech-
nology in a tiny package, precluding the need to call on
applications specialists when a project is underway. All
internal components are optimized for the highest system
performance, with integration and layout issues resolved
in the package. The LTM9001 is a tested, individual pack-
age that can be picked and placed easily on the board,
thus reducing the required design time and complexity
normally associated with such functions.
In addition, the LTM9001 has the potential for custom-
ization. For orders meeting a minimum size, the LTM9001
can be conigured for various sampling rates and the
differential ADC driver can be substituted for ixed gain
versions ranging from 8dB up to 26dB. As a result, the
LTM9001 signiicantly eases the challenge in designing
high performance communications and instrumentation
systems.
EDN Innovation Award Finalists
EDN magazine in January announced inalists for the
annual EDN Innovation Awards, which includes several
Linear Technology nominees.
q For Innovator of the Year, EDN nominated Linear co-
founder and Chief Technology Oficer Robert Dobkin.
q In the Power ICs category, EDN selected as a inalist
the LT3080 3-terminal adjustable LDO regulator,
which was designed by Bob Dobkin and his team at
Linear Technology.
q For the Analog IC category, the LTC6102 current
sense ampliier.
q For Best Contributed Article, Jim Williams’ article,
“Designing Instrumentation Circuitry with RMS/DC
Converters.”
Visit www.linear.com for complete descriptions and
data sheets for these products. See www.edn.com for Jim
Williams’ article.
On the Road in China
Linear Technology is on the road in China, exhibiting in the
4-city IIC Conference & Exhibition. Linear is participating
with a booth at all four conference locations:
q Chengdu—February 28–29, Booth 5D32
q Shenzhen—March 3–4, Booth 2H06
q Beijing—March 6–7, Booth B17
q Shanghai—March 10–11, Booth 4Q09
At the IIC Conference, where overall attendance is expect-
ed to exceed 30,000, Linear will highlight a broad range of
products and solutions. These include Power Management
ICs (PMICs), power µModule controllers, LT3080 3-termi-
nal linear regulator, high speed ADCs, the new LTM9001
µModule receiver, high frequency RF products including the
LT5570 RMS
power detector,
the LTC6102
current sense
ampliier, ADC
drivers, DACs,
and LED driv-
ers. L
Linear Technology
will highlight a
broad range of
products at the
IIC Conference
with a booth at all
four conference
locations in China.
2
Linear Technology Magazine March 2008
2
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DESIGN FEATURES L
LTM9001, continued from page 1
Figure 1. A typical application and simplified block diagram of the LTM9001
signals with an intermediate frequency
(IF) range up to 300MHz. Figure 1
shows a typical application.
How is a µModule component
different than a traditional IC? The
µModule construction allows the
LTM9001 to mix standard ADC and
ampliier components regardless of
their process technology and match
them with passive components for
a particular application. The result
is a high performance product with
no process technology compromises
and the potential for semi-custom
adaptations.
Extracting the full performance
from 16-bit, high speed ADCs requires
careful layout as well as good circuit
design. The substrate design carefully
shields sensitive analog traces, maxi-
mizes thermal conduction through
multiple ground pads and minimizes
coupled noise by including bypass
capacitors inside the module and close
to the ADC. A common problem with
traditional ADC board layouts is long
traces from the bypass capacitors to
the ADC. The bare die construction
with internal bypass capacitors pro-
vides the closest possible decoupling
and eliminates the need for external
bypass capacitors.
The passive ilter network imple-
ments an antialias ilter and matches
the ampliier outputs to the ADC in-
puts. Most communications receiver
applications utilize a highly selective
ilter between the mixer and the ADC
driver. The antialias ilter between
the ADC driver and the ADC inputs
limits the wideband ampliier noise
and helps preserve the high SNR of
the ADC. Printed circuit board (PCB)
layout has a signiicant impact on the
performance even if the circuit topol-
ogy and component values are correct.
The signal paths must be symmetric
and isolated from the clock inputs and
digital outputs.
The low noise, low distortion ampli-
ier stage provides gain without adding
signiicant noise or distortion to the
signal. Despite the low noise of the
ampliier, the noise is multiplied by the
same gain as the ampliier, so higher
gain unavoidably adds noise to the
system. However, the input range of
the ampliier is proportionately smaller
thanks to the gain and this smaller
input range allows for lower distortion
from the preceding components. The
ampliier inputs present a resistive
200 Ω differential input impedance
which is simple to match to most
common, high speed, single-ended or
differential signal paths. This presents
a more straightforward interface than
a switched-capacitor ADC and simpli-
ies the connection to the inal stage
of the RF signal chain.
What’s Inside?
The µModule receiver consists of wire-
bonded die, packaged components and
passives mounted on a high perfor-
mance, 4-layer, Bismaleimide-Triazine
(BT) substrate. BT is similar to other
laminate substrates such as FR4 but
has superior stiffness and a lower
coeficient of thermal expansion.
In time, several different versions
of the LTM9001 will be available. The
LTM9001-AA, as the irst release, is
conigured with a 16-bit, 130Msps
ADC. The ampliier gain is 20dB with
an input impedance of 200 Ω and an
input range of ±250mV. The match-
ing network is designed to optimize
the interface between the ampliier
outputs and the ADC inputs under
these conditions. Additionally, there
is a second order bandpass ilter
designed for 162.5MHz, ±25MHz to
prevent aliasing and to limit the noise
from the ampliier.
Why 162.5MHz?
The ADC inside the LTM9001 has
a full power bandwidth of 700MHz
and the ampliier is suitable for in-
put frequencies up to 300MHz, so
why was 162.5MHz chosen for this
irst version? Nyquist theory tells us
that the minimum sample rate for a
given input frequency is twice that
frequency. Working backwards, an
ADC sampling at 130Msps can cap-
ture a frequency range up to 65MHz
wide. Undersampling allows us to
move that frequency range. Hence
the irst Nyquist zone is DC – 65MHz,
the second is 65MHz to 130MHz, the
third is 130MHz to 195MHz, and so
on, see Figure 2.
The LTM9001-AA is intended for
instrumentation applications. In such
applications, the linearity and dynamic
range requirements are extremely
high. Traditional instruments utilize
preselectors and multiple down-con-
Linear Technology Magazine March 2008
3
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L DESIGN FEATURES
Figure 2. Nyquist zones for 130MHz sample rate
version stages to place the band of
interest at DC. With the advent of high
performance ADCs capable of unders-
ampling, modern instruments are able
to eliminate the inal down-conversion
stage without sacriicing performance.
The LTM9001-AA coniguration se-
lects the third Nyquist zone with the
bandpass ilter set squarely in the
middle of the zone.
The signal must be iltered and ampli-
ied through each stage. Ampliication
(gain) increases the total noise and
reduces the headroom, which gener-
ally causes more distortion. The added
distortion may be addressed with a
higher supply voltage or a higher power
ampliier, neither of which is prefer-
able. Therefore, from the system-level
point of view, an ADC with a small
input range is better.
The LTM9001 meets these system-
level criteria. The resistive ampliier
inputs are easily matched and it has
an input range of ±250mV, enabling
the use of low OIP3 components or
higher loss SAW ilters. The noise of
the ampliier is low enough that the
SNR of the LTM9001 is good despite
the high gain (see Figure 3).
output ground (OGND) are electrically
isolated on the LTM9001, but for most
digital output conigurations should
be connected on the PCB underneath
the part to provide a common return
path.
Use multiple ground vias. Using as
many vias as possible helps to improve
the thermal performance of the board
and creates necessary barriers sepa-
rating analog and digital traces on the
board at high frequencies. Take care to
separate analog and digital traces as
much as possible, using vias to create
high frequency barriers. This reduces
digital feedback that can reduce the
signal-to-noise ratio (SNR) and dy-
namic range of the LTM9001.
More than Just
a Buffered ADC
The sample-and-hold front end of
discrete ADCs presents a complex
charge/discharge proile to the drive
circuitry. Ideally, the input circuitry
should be fast enough to fully charge
the sampling capacitor during the
sampling period (half of the clock
period), but this is not always pos-
sible and the incomplete settling may
degrade the SNR and SFDR. Some
manufacturers promote a “buffered”
ADC as a solution but this falls short
of addressing the system-level solution
since a low distortion ampliier is still
required to provide the full-scale input
to the ADC.
From the system view, the ADC
follows the RF and IF portions of the
receiver chain and converts the signal
to a digital format. The signal comes
from the antenna with very little power.
Conclusion
µModule technology, introduced irst
by Linear Technology for DC/DC con-
verters, now brings the advantages of
small size, higher integration and ease
of use for high speed ADC applications.
By integrating ine-line CMOS and
SiGe components with appropriate
passive networks, the challenging
task of matching a ixed gain ampli-
ier to a high speed ADC is done. All is
reduced to an easy-to-use black box:
the LTM9001. L
Working with a
µModule Receiver
The LTM9001 uses a land grid array
(LGA), which provides higher pin den-
sity than dual in-line or quad packages
and better thermal conduction than
BGA packages. The high integration
of the LTM9001 makes the PCB board
layout simple. The multilayer substrate
allows greater lexibility in pin place-
ment on the package relative to pin
placement on the die. The LTM9001
has been optimized for a low-through
layout so that the interaction between
inputs, clock and digital outputs is
minimized. The analog and clock in-
puts are surrounded by ground pads
and a continuous row of ground pads
further separate the analog and digital
signal lines. However, to optimize its
electrical and thermal performance,
some layout considerations are still
necessary. See the actual evaluation
board in Figure 4.
Use large PCB copper areas for
ground. This helps to dissipate heat
through the board and also helps
to shield sensitive on-board analog
signals. Common ground (GND) and
Figure 3. An FFT of the LTM9001 at 160MHz
input frequency with the randomizer on
Figure 4. An evaluation board shows the
small overall circuit. Note that no external
components are required.
4
Linear Technology Magazine March 2008
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DESIGN FEATURES L
Voltage and Current Monitoring from
7V to 80V in 3mm × 3mm DFN-10
by Zhizhong Hou
Introduction
Accurate power supply voltage and
current monitoring is increasingly
important in everything from in-
dustrial and telecom applications to
automotive and consumer electronics.
A complete power monitoring system
typically includes a sense resistor, a
precision ampliier, an analog to digital
converter (ADC) and a proper interface
to report data to a host controller. The
LTC4151 and LTC4151-1 combine all
of these components (except the sense
resistor) into one IC, resulting in a full
featured, rugged and simple-to-use
solution for accurate high side cur-
rent sensing and voltage monitoring
(see Figure 1).
The LTC4151 and
LTC4151-1 offer the benefits
of high side current sensing
without any of the usual
complexity. Each integrates
a precision high voltage
amplifier and associated
level shift circuit for high
side current sensing, a
precision voltage divider for
supply voltage monitoring,
a 12-bit ADC and an I 2 C
interface—all in small MS10
or tiny 3mm × 3mm
DFN-10 packages.
with changing load current. This can
result in the load seeing signiicant
ground noise during transient spiking
load currents. Worse yet, a failed or
disconnected low side sense resistor
causes the load ground to be charged
to the full supply voltage, presenting
a potential safety hazard.
High side sensing avoids these prob-
lems, but requires a number of high
performance devices and interfaces.
For instance, a robust high side sense
ampliier is required to withstand
high supply voltage or high voltage
transients. Also, a precision level shift
circuit is needed to accurately trans-
late the large supply-referred signals
to appropriate ground level signals
for the ADC.
High Side vs
Low Side Sensing
In a power monitoring system, the
sense resistor can be placed either
between the system ground and the
load (low side sensing) or between
the system supply and the load (high
side sensing). For many applications,
high side sensing is desirable, but
it is traditionally more dificult to
implement.
Low side sensing is relatively simple
in concept and design, but a low side
sense resistor loats the load above
system ground. Thus, the ground
potential seen by the load varies
Full Featured High Side,
High Voltage Digital Monitors
The LTC4151 and LTC4151-1 offer the
beneits of high side current sensing
without any of the usual complex-
ity, plus they provide supply voltage
monitoring in the same package. Each
Ω
LTC4151
SHDN
50
Figure 1. Full featured current and voltage monitor simplifies high voltage, high side sensing.
Linear Technology Magazine March 2008
5
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