LT6552
1
DESCRIPTION
Demonstration circuit 815 is a Video Line-Driver & Re-
ceiver featuring the LT6552. The driver and receiver sec-
tions of the board are scored to be separable by the user
so that evaluation with an intermediate cable-run is read-
ily accomplished. Figure 1 shows the general layout of
the board and the usual connections used to perform
evaluations.
The intermediate cable link may be either 75
Ω
coaxial-
cable (such as RG-6) or twisted-pair (CAT-5 Ethernet ca-
ble). BNC connectors are provided for the coaxial con-
nections, including those for the source and destination
video gear. The twisted-pair option for the intermediate
link uses RJ-45 connections and transports over con-
tacts 7 & 8 (this pair is generally not used in Ethernet
datacom applications and is always straight-through).
Jumpers are provided in the receive section that offer 4
levels of cable-loss equalization, sufficient for CAT-5 ca-
ble-runs up to 1000 feet (or 11dB max @ 4MHz).
The Performance Summary table below indicates the key
electrical characteristics provided by this evaluation
board.
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PERFORMANCE SUMMARY
Specifications are at T
A
= 25°C
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
S
Power Supply Range No clipping of ac-coupled video 4.0 5.0 12.6 V
I
S
Power Supply Current 5V, video active, TX & RX total 40 mA
Z
INTX
Video Input Impedance V
IN
= 1.0V
P-P
75
Ω
Z
OUTRX
Video Output Impedance V
OUT
= 1.0V
P-P
75
Ω
Z
OUTTX
Line-driver Output Impedance V
OUTTX
= 1.0V
P-P
unbalanced coax
V
OUTTX
= 1.0V
P-P
twisted-pair
75
110
Ω
Z
INRX
Line-Receiver Input Impedance RX Zin jumper set to 75 ohm
RX Zin jumper set to 110 ohm
75
110
Ω
f
CU
Frequency Response, upper –3dB any EQ setting
10 MHz
f
CL
Frequency Response, lower –3dB 5.5 Hz
EQ
Line-Receiver Equalization,
Set by Jumpers EQ1 & EQ2
EQ1=OFF, EQ2=OFF (none)
EQ1=ON, EQ2=OFF, f=3.58MHz (
≈
300’)
EQ1=OFF, EQ2=ON, f=3.58MHz (
≈
800’)
EQ1=ON, EQ2=ON, f=3.58MHz (
≈
1000’)
0
3.0
9.0
10.5
dB
OPERATING PRINCIPLES
The LT6552 in location U1 is used as a line-driver that
accepts a differential input and generates a single-ended
output in a gain-of-two configuration. The input provided
is a 75
Ω
un-balanced connection that may contain sub-
stantial common-mode noise. The output is back-
terminated to drive either a purely unbalanced 75
Ω
coax-
ial link, or a quasi-balanced 110
Ω
CAT-5 link. The simpli-
fied CAT-5 output as implemented here will produce a
DEMO CIRCUIT 815
QUICK START GUIDE
LT6552
3.3V Single Supply Video
Difference Amplifier
LT6552
2
video common-mode on the pair that may be undesirable
in a productized realization, but helps demonstrate the
capability of the receiver function of the DC815. U1 also
demonstrates the compact DFN package.
The LT6552 in the U2 location (in the standard SO-8
package) is set up similarly, except that equalization net-
works are added to provide cable-loss correction. The
input in this receiver circuit may be terminated in either
75 or 110 ohms, depending on the link media used. The
output is an unbalanced 75
Ω
output, suitable for driving
standard video equipment. The maximum equalization
circuitry is optimized for 1000’ (300m) CAT-5 cable, pro-
viding over 10dB of gain boost at the color sub-carrier
frequency (i.e. 3.58MHz) as shown in Figure 2.
AC-coupling on the inputs and outputs allows the circuitry
to operate from single supplies down to 4V (note that in
dc-coupled video applications, the part is useful down to
3V). The supply and ground connections for the TX and
RX sections are completely isolated so that common-
mode ground-noise may be introduced during evaluation
(up to 1.5V
PP
with 5.0V supplies). The supply current for
each section is typically 20mA with active video present.
Bringing the Shutdown connection(s) to ground will bring
the LT6552(s) into a low-power idle condition.
The Bill-of-Materials is shown in Figure 3 and the sche-
matic diagram of the circuitry in Figure 4.
QUICK START PROCEDURE
Demonstration circuit 815 is easy to set up to evaluate the
performance of the LT6552. Refer to Figure 1 for proper
measurement equipment setup and follow the procedure
below:
1.
Place jumpers in the following positions:
EQ2 “OFF” position
EQ1 “OFF” position
RX Zin “110 ohm” position
2.
With power off, connect the power supply to +5V TX
and +5V RX, and the supply return to GND TX and GND
RX.
3.
Connect a 75
Ω
video source to TX IN (J2), such as a
camera or pattern generator, and assure that the signal
source is powered and active.
4.
Connect a 75
Ω
video monitor or signal analyzer to RX
OUT (J6), assuring that the device is powered and
ready to accept video.
5.
Connect a CAT-5 patch cable from TX OUT (J1) to RX
IN (J5).
6.
Turn on the demo circuit power supply.
Note.
Make sure that the input voltage does not exceed 12V. All four board
connections must be made to energize the entire circuitry.
7.
Check for the proper RX OUT output signal. The output
video waveform should be a high fidelity copy of the
original input (ac-coupled however).
Note.
The output must be properly loaded with 75
Ω
, otherwise the signal
will appear doubled in amplitude.
8.
If longer CAT-5 cables are available, such as a full
1000’ (300m) carton with RJ-45 connectors fitted, the
EQ1 and EQ2 jumpers may be repositioned for best
video reproduction.
9.
Evaluation may also be performed with 75
Ω
coaxial-
cable as the intermediate link instead of the CAT-5
twisted pair. To do this, remove the CAT-5 cable, move
the JP3 jumper (RX Zin) to the “75 ohm” setting, and
connect the coax link from TX OUT (J3) to RX IN (J4).
Set the EQ1 and EQ2 jumpers for the best video repro-
duction.
10.
In some instances, it may be desirable to locate the TX
and RX sub-circuits remote from one another to test an
actual link, such as in an elevator car. The DC815 is
scored to allow breaking the board into two standalone
sections in order to place them separately. For short-
term tests, a 9V battery may be used as a power
source for each individual section, should powering be
inconvenient otherwise.
JP1
JP2
JP3
LT6552
3
Video
Source
Video
Monitor
Power
Supply
–
+
CAT 5 cable,
up to 1000 ft.
(300m)
Figure 1.
Proper Measurement Equipment Setup
Figure 2.
Receive Equalization Characteristic (max)
LT6552
4
Figure 3.
DC815 Bill-of-Material
LT6552
5
Figure 4.
DC815 Electrical Schematic Diagram
