November 2004 - Superfast Fixed-Gain Triple Amplifiers Simplify Hi-Res Video Designs

DESIGN FEATURES
Superfast Fixed-Gain Triple Amplifiers
Simplify Hi-Res Video Designs by Jon Munson
Introduction
5V
MARKER: –0.5dB = 466.771638851MHz
10dB/DIV
1MHz
10MHz
100MHz
Figure 2. Wide frequency response
of circuit in Figure 1
12
1GHz
16
LT6553
2
3
RIN
15
+
75Ω
14
–
75Ω
370Ω
75Ω
370Ω
4
13
370Ω
5
GIN
75Ω
6
75Ω
–5V
–5V
75Ω
12
+
370Ω
7
BIN
370Ω
–
370Ω
11
–
10
75Ω
5V
75Ω
+
75Ω
9
8
–5V
Figure 1. LT6553 RGB cable driver circuit
of baseband video generally require reproduction of high-frequency content
up to at least the 5th harmonic of the
fundamental frequency component,
which is 2.5 times the video pixel
rate, accounting for the 2 pixels per
fundamental cycle relationship. This
indicates that for UXGA in particular,
flat frequency response to beyond
0.5GHz is required!
Easy Solution for MultiChannel Video Applications
Baseband video generated at these
higher rates is processed in either native red-green-blue (RGB) domain or
encoded into “component” luma plus
blue-red chroma channels (YPbPr);
three channels of information in either
case. With frequency response requirements extending to beyond 500MHz,
amplifier layouts that require external
resistors for gain setting tend to waste
valuble real-estate, and frequency
response and crosstalk anomalies can
plague the printed circuit development
process.
The LT6553 and LT6554 conveniently solve all these problems by
providing internal factory-matched
resistors and an efficient 3-channel
flow-through layout arrangement
using a compact SSOP-16 package.
Figure 1 shows the typical RGB cable
driver application of an LT6553, and
its excellent frequency and time response plots are shown in Figures 2
and 3. Frequency markers in Figure
2 show the –0.5dB response beyond
450MHz and –3dB response at about
600MHz.
What’s Inside
The LT6553 and LT6554 integrate
three independent sections of circuitry
that form classic current-feedback
amplifier (CFA) gain blocks, all
implemented on a very high-speed
fabrication process. The diagram in
Figure 4 shows the equivalent internal
circuitry (one CFA section shown).
1.5
VIN = 1VP–P
VS = 5V
1.0 RL = 150Ω
TA = 25°C
0.5
OUTPUT (V)
The LT6553 and LT6554 triple video
amplifiers offer 600MHz performance
in a compact package, requiring no
external gain-setting resistors to
establish gain of 2 or unity-gain, respectively. One may wonder “Why are
such super-fast amplifiers are now
necessary in video designs—isn’t that
overkill?” The answer is a resounding
no. The proliferation of high-resolution
video displays, both in the professional
and consumer markets has markedly
increased the analog bandwidth of
baseband video signals. The latest
demands of video equipment are so
far ahead of the last generation that
the performance of the LT6553 and
LT6554 is not overkill at all, but in
fact mandatory.
For example, digital studio equipment for NTSC broadcast television
typically uses pixel-rates around 14
million per second, while now ubiquitous XGA computer outputs (1024
x 768) routinely churn out about 80
Megapixels per second. The latest High
Definition consumer formats put out
a comparable 75Mpixel stream and
the increasingly popular UXGA professional graphics format (1600 x 1200)
generates a whopping 200Mpixel per
second flow. Obviously the accurate
video reproduction of these newer formats is placing exceptional demands
on the frequency response of the
video amplifiers involved. Specifically,
pulse-amplitude waveforms like those
1
0
–0.5
–1.0
–1.5
0
2
4
6
8 10 12 14 16 18 20
TIME (ns)
Figure 3. Fast pulse response
of circuit in Figure 1
Linear Technology Magazine • November 2004
DESIGN FEATURES
V+
V+
TO OTHER
AMPLIFIERS
BIAS
AGND
370
V+
46k
EN
1k
IN
150
370
OUT
V–
DGND
V–
V–
Figure 4. LT6553 & LT6554 simplified internal circuit functionality
The on-chip feedback resistors set
the closed-loop gain to unity or two,
depending on the part. The nominal
feedback resistances are chosen to
optimize the frequency response for
maximal flatness under the anticipated loading conditions. The LT6553
is intended to drive back-terminated
50Ω or 75Ω cables (for effective loading of 100Ω to 150Ω respectively),
while the LT6554 is useful for driving
ADCs or other high impedance loads
(characterized with 1kΩ as a reference
loading condition).
All three CFAs have a bias control
section with a power-down command
input. The shutdown function includes
internal pull-up resistance to provide a
default disable command, which when
invoked, reduces power consumption
to less than 100µA for an entire threechannel part. During shutdown mode
the amplifier outputs become high
impedance, though in the case of the
LT6553, the feedback resistor string
to AGND is still present. The parts
come into full-power operation when
the enable input voltage is brought
3.3V
NC7SZ14
1
LT6554
2
3
R1
15
×1
4
5
G1
B1
75Ω
75Ω
75Ω
14
13
×1
6
7
16
12
11
×1
8
10
9
ROUT
GOUT
1
SEL
R0
15
3
14
5
6
7
B0
75Ω
75Ω
75Ω
16
2
4
G0
LT6554
8
×1
×1
×1
BOUT
13
MUXing Without Switches
RGB and YPbPr video signals are commonly multiplexed (selections made
on an occasional basis) to reduce I/O
connector count or otherwise re-use
various high-value video signalprocessing sections when selecting
various modes of operation in the
end use of the product. This has often
been accomplished with the use of FET
switches and buffer amps to route
the various video channel signals,
but can alternatively be performed by
use of the power-down functionality
included in the LT6553 and LT6554.
Figure 5 shows an example circuit
using LT6554 units cross-controlled
to allow a single video path to be
enabled at any particular time. This
might be the situation at the input
side of a video display or AV receiver
continued on page 36
VIN
3V TO 5.5V
CIN
10µF
OFF ON
12
VOUT
VIN
LTC1983-3
SHDN
VOUT = –3V
IOUT = UP TO 100mA
COUT
10µF
GND
C–
C+
11
10
9
NOTE:
POWER SUPPLY BYPASS
CAPACITORS NOT SHOWN FOR CLARITY
–3.3V
Figure 5. Video input multiplexer using LT6554 shutdown feature
Linear Technology Magazine • November 2004
within 1.3V above the DGND pin.
The typical on-state supply current of
8mA per amplifier provides for ample
cable-drive capacity and ultra-fast
slew rate performance of 2.5V per
nanosecond!
CFLY
1µF
CFLY: TAIYO YUDEN LMK212BJ105
CIN, COUT: TAIYO YUDEN JMK316BJ106ML
Figure 6. Generating a local –3V
supply with 4 tiny components
13
DESIGN IDEAS
Figure 6 shows a solution with an
optimization to provide a wide asymmetric common-mode range (–12V
to 73V) as might be encountered
in an automotive environment. The
amplifier is biased from just a single
+5V power supply. The asymmetry
of the common-mode window is controlled by the applied VREF voltage,
provided here by a versatile LT6650
resistor-programmable reference (see
+VSOURCE
article in this issue: ‘Tiny, ResistorProgrammable, µPower 0.4V to 18V
Voltage Reference’). The LT1990 is
shown strapped to produce a gain of
ten and outputs a bidirectional signal
referenced around VREF. The excellent
CMRR of the LT1990 keeps output
ripple from the H-bridge PWM activity
at a low level so that simple filtering
(not shown) can accurately recover the
desired low-frequency motor current
information.
5V
Conclusion
These three new amplifiers are so
versatile and easy to use, it is possible
to stock one of them and use it for
many varied applications. No external
components are needed to achieve
hundreds of gains in non-inverting,
inverting, difference and attenuator
configurations. Just strap the pins
and go. It’s a great way to reduce
inventory, ease manufacturing, and
simplify a bill of materials.
LT1990
10k
900k
8
7
– +
2
1M
3
1M
RS
6
+
VREF = 1.5V
IL
100k
–
4
IN
OUT
LT6650
GND FB
10k
1nF
54.9k
40k
40k
900k
5
VOUT
For RS = 1mΩ:
VOUT = 0.5V for IL = 100A
VOUT = 1.5V for IL = 0A
VOUT = 2.5V for IL = –100A
100k
20k
–12V VCM 73V
VOUT = VREF ± (10 • IL • RS)
1
1µF
Figure 6. Sensing current in a bidirectional full bridge motor
LT6553/4, continued from page 13
that requires selecting between a set
of RGB or component video sources.
A similar circuit using LT6553s provides a means of output selection as
might be the case in a video recorder
where switching between live feed and
playback would be needed.
Operating With the
Right Power Supplies
The LT6553 and LT6554 require a
total power supply of at least 4.5V, but
depending on the input and output
swings required, may need more to
avoid clipping the signal. The LT6554,
having unity gain, makes the analysis
simple—the output swing is about
(V+ – V-) – 2.5V and only governed by
the output saturation voltages. This
means a total supply of 5V is adequate
for standard video (1VP–P). For the
LT6553, extra allowance is required
for load-driving, so the output swing
36
is (V+ – V–) – 3.8V. This means a total
supply of about 6V is required for the
output to swing 2VP–P, as when driving
cables. For best dynamic range along
with reasonable power consumption, a
good choice of supplies would be ±3V
for the LT6554 and 5V/–3V for the
LT6553. Since many systems today
lack a negative supply rail, a small
LTC1983-3 solution can be used to
generate a simple –3V rail for local use,
as shown in Figure 6. The LTC19833 solution is more cost effective and
performs better than AC-coupling
techniques that might otherwise be
employed.
operation. DC743A includes biasing
and AC-coupling components with
the LT6553 in a single supply configuration. DC794A is identical to
the DC714A except it has the LT6554
installed. All three of these demo
circuits have high-quality 75Ω BNC
connections for best performance
and include a calibration trace to allow connector effects to be removed
from network analyzer sweeps of the
amplifier under evaluation. The demo
circuits also illustrate high-frequency
layout practices that are important to
realizing the most performance from
these super-fast parts.
Demo Circuits Available
Demonstration boards that use the
LT6553 and LT6554 are available to
simplify evaluation of these parts. To
evaluate the LT6553 ask for DC714A
or DC743A. DC714A is a DC-coupled
circuit that is intended for split supply
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • November 2004