LINEAR TECHNOLOGY LTM9002 Technical data

L DESIGN FEATURES

OF

GND

LO

VCC= 3V VDD= 3V

DIFFERENTIAL

AMPLIFIERS

MAIN

RF

INA

–

INA

+

V

REF

OV

DD

0.5V TO V

DD

SAW

FILTER

MUX

CLKOUT

ADC CLK
SPI

14-BIT

125Msps ADC

LO

DIVERSITY

RF

INB

–

INB

+

OGND

SAW

FILTER

14-BIT

125Msps ADC

DAC

DAC

Complete Dual-Channel Receiver
Combines 14-Bit, 125Msps ADCs,
Fixed-Gain Amplifiers and Antialias
Filters in a Single 11.25mm × 15mm
μModule Package

Introduction

Extensive hands-on applications
experience is a prerequisite for any
designer hoping to take full advan­tage of an ADC’s capabilities when
sampling high dynamic range signals
in multichannel IF-sampling or in I/Q
baseband communications channels.
An intimate knowledge of the ampli­fier output stage and ADC front end
is required to match the impedances,
while careful attention to layout is
required to minimize coupling of
the digital outputs into the sensitive
analog input.

In fact, good layout is paramount
to maintaining ADC performance,
yet ever more demanding market
requirements call for smaller designs
and higher channel density, which
exacerbate layout issues. These
design requirements can challenge
even the most seasoned engineer if

his expertise lies in the RF or digital
worlds.

The LTM9002 dual-channel, IF/
baseband receiver harnesses years
of applications design experience
and squeezes it into an easy-to-use

11.25mm × 15mm µModule package.
Inside the package is a high perfor­mance dual 14-bit ADC sampling up
to 125Msps, antialiasing filters, two
fixed gain differential ADC drivers and
a dual auxiliary DAC. By combining
these components, the LTM9002 elimi­nates the burden of input impedance
matching, filter design, gain/phase
matching, isolation between channels
and high frequency layout, dramati­cally improving time to market. Even
in this small package, the LTM9002
guarantees high performance that will
enhance many communications and
instrumentation applications.

by Todd Nelson

Multichannel
ADC Applications

Multichannel applications have sev­eral unique requirements, such as
channel matching in terms of gain,
phase, DC offset, and channel-to­channel isolation. Gain and phase
errors directly affect the demodula­tion of the I and Q channels. And
since direct conversion receivers
are typically DC-coupled, DC offset
limits the dynamic range of the re­ceiver. Multiple-input, multiple-output
(MIMO) systems depend on multiple
receiver channels all receiving the
same signal while detecting the slight
variations caused by multipath delays
so gain and phase errors affect these
systems as well. Like I/Q receivers,
diversity receivers require excellent
isolation between channels because
crosstalk appears as noise corruption
and can be more difficult to suppress

14

Figure 1. LTM9002 used for a Main/Diversity receiver.

Linear Technology Magazine • June 2008

DESIGN FEATURES L

FFT VALUE (dBFS)

FFT POINT

81921

0

–120

1024 2048 3072 4096 5120 6144 7168

–10

–110

–100

–90

–80

–70

–20

–30

–40

–50

–60

HD3

HD2

REF

BUFFER

10k

1.25V

1V (OPEN CIRCUIT,

4k THEVENIN
RESISTANCE)

10k

1000pF

2.49k

SENSE

RANGE

SELECT

REF

1.5V

REFERENCE

LTM9002

DAC

SDI

CS/LD

SCK

with digital filtering. Clearly, channel
matching and channel isolation of
the ADC and driver circuits directly
impact system-level performance.
For many multichannel applications,
these errors cannot be corrected in
the digital domain.

Performance

The LTM9002 achieves 66dB Signal
to Noise Ratio (SNR) and 74dB Spu­rious Free Dynamic Range (SFDR) at
140MHz input frequency. Figure 2
shows the FFT under these conditions.
SNR is a function of the ADC and
amplifier performance as well as the
amplifier gain and filter bandwidth.
The inherent amplifier noise is pro­portional to the voltage gain; therefore,
the 26dB amplification increases the
amplifier noise by 20 times whereas
8dB amplification would only increase
the noise by 2.5 times. Likewise, the
amplifier noise (in nV/√Hz) increases
with the square of the filter bandwidth.
It is important to remember these re­lationships when assessing the entire
signal chain.

For multichannel applications,
channel-to-channel matching and
isolation are important considerations.
The LTM9002 achieves 90dB isolation
at 140MHz input frequency despite
the small form factor. The overall gain
is typically 26dB on the default span
setting and varies just 0.1dB between
the two channels. The 12-bit auxiliary
DAC can be configured to adjust the
span by 61µV per step using the circuit
in Figure 3.

Another important performance
metric is printed circuit board (PCB)
area efficiency. Here, the LTM9002
excels. The LTM9002 requires no ex­ternal components—no supply bypass
capacitors, no passive filtering, no
impedance matching or translating
components. In many IF-sampling
applications, gain can be obtained
through transformers, but they are
often large and difficult for automated
assembly equipment to mount. In
DC-coupled applications, amplifiers
are required as ADC drivers, along
with their associated antialias filter
network. It is not uncommon for the
entire IF/baseband receiver system to

Linear Technology Magazine • June 2008

Figure 2. FFT showing LTM9002 AC
performance with 140MHz input frequency.

consume two square inches of board
area (approximately 25mm × 50mm).
None of this external circuitry is re­quired with the LTM9002 so it requires
only about one-quarter of a square
inch (11.25mm × 15mm), better by a
factor of eight.

Attributes and Configurations

The µModule construction allows the
LTM9002 to mix standard ADC and
amplifier components regardless of
their process technology and match
them with passive components for a
particular application. The µModule
receiver consists of wire-bonded die,
packaged components and passives
mounted on a high performance,
4-layer, Bismaleimide-Triazine (BT)
substrate. BT is similar to other lami­nate substrates such as FR4 but has
superior stiffness and a lower coef­ficient of thermal expansion.

The LTM9002-AA utilizes a dual, 14­bit, 125Msps ADC, two 26dB fixed-gain
amplifiers and also includes a 12-bit
dual DAC configured for full-scale
span adjustment as shown in Figure 1.
Internal antialias filters limit the input
frequency to less than 170MHz. The
amplifiers present a 50Ω differential
input impedance and an input range
of ±50mV, or –16dBm. This default
span is set by connecting the SENSE
pin to VDD, and can be adjusted in
three ways. For a –3dBm lower span,
the SENSE pin can be connected to

1.5V. By connecting SENSE to VDD or

1.5V, the internal reference is used.
An external reference can be used by
applying 0.5V–1.0V to the SENSE pin.
The auxiliary DAC offers a final option
for selecting the range. Alternately,
fine adjustments to the span, such as
balancing the gain of the two channels,
can be made with external references
or the auxiliary DAC.

Multiple power saving modes in­clude independently disabling either
amplifier or the ADC. The ADC has
two shutdown states: NAP and SLEEP
modes. In NAP mode, the internal
reference remains biased so that
conversions can resume within 100
clock cycles upon start-up. In SLEEP
mode the reference is shut down and
start-up takes 1µs or more. A clock
duty cycle stabilizer feature is avail­able and an output clock signal is
provided for accurately latching the
output data. The two channels can be

Figure 3. Using an external reference and the internal auxiliary DAC for span adjustment.

15