Texas Instruments | AN-2156 Configuring the LMH0387 for an Equalizer-Only Application (Rev. A) | Application notes | Texas Instruments AN-2156 Configuring the LMH0387 for an Equalizer-Only Application (Rev. A) Application notes

Texas Instruments AN-2156 Configuring the LMH0387 for an Equalizer-Only Application (Rev. A) Application notes
Application Report
SNLA130A – July 2011 – Revised April 2013
AN-2156 Configuring the LMH0387 for an Equalizer-Only
Application
.....................................................................................................................................................
ABSTRACT
This application note discusses how to configure the LMH0387 for an equalizer-only application.
1
2
3
4
5
Contents
Introduction ..................................................................................................................
How to Configure the LMH0387 for Equalizer-Only Use ..............................................................
LMH0387 Benefits over Dedicated Equalizer ...........................................................................
3.1
Superior Return Loss ..............................................................................................
3.2
Reduced BOM Costs ..............................................................................................
3.3
Simplified Layout and PCB Space Savings ....................................................................
Conclusion ...................................................................................................................
References ...................................................................................................................
2
2
3
3
4
4
5
5
List of Figures
1
LMH0387 Circuit for Equalizer-Only Mode .............................................................................. 3
2
LMH0387 Input Return Loss............................................................................................... 4
3
Input Circuit Required for Dedicated Equalizer (left) and LMH0387 (right)
4
Input Circuit PCB Layout for Dedicated Equalizer (left) and LMH0387 (right) ...................................... 5
.........................................
4
List of Tables
1
LMH0387 Connections for Equalizer-Only Mode ....................................................................... 2
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SNLA130A – July 2011 – Revised April 2013
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AN-2156 Configuring the LMH0387 for an Equalizer-Only Application
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1
Introduction
1
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Introduction
The adaptive cable equalizer is an important component in the Serial Digital Interface (SDI) used for
transmission of digital video signals. It restores the SDI signal after it passes through hundreds of meters
of coaxial cable, automatically adjusting to reverse the effects of the cable loss. The cable driver’s role is
to set the appropriate amplitude, slew rate, and eye shape to drive an SDI signal into the coaxial cable so
that it can be properly recovered by the equalizer at the other end.
The LMH0387 SDI configurable I/O accomplishes both of these tasks with a single pin, allowing a BNC
attached to the device to be used as either an input or output. It provides the full functionality of an
adaptive cable equalizer and cable driver. The LMH0387 includes an integrated return loss network which
enables very good return loss and easy compliance to the Society of Motion Picture and Television
Engineers (SMPTE) return loss specifications. The versatility of the LMH0387 allows it to replace an
equalizer or cable driver (or both) while providing several additional benefits.
2
How to Configure the LMH0387 for Equalizer-Only Use
In sockets that only require the equalizer function (input only), the LMH0387 may be configured as an
equalizer by disabling the internal cable driver. One method to do this is to set the device per the typical
application (see the datasheet) with the TX_EN pin tied low to disable the cable driver. While disabled, the
cable driver consumes a small amount of power (4 mW). With this configuration, the cable driver can be
enabled to change the port from input to output by pulling the TX_EN pin high.
Alternatively, if the cable driver will never be used, the LMH0387 can be configured for equalizer-only
mode (with no cable driver option) by connecting all pins associated with the cable driver to ground. This
saves components, reduces power consumption, and simplifies the PCB layout.
To configure the LMH0387 for equalizer-only use in this manner, connect the following nine LMH0387 pins
to ground as shown in Table 1. The cable driver will consume no power and only the equalizer portion of
the LMH0387 will be active. Figure 1 shows the complete LMH0387 circuit for equalizer-only mode.
Table 1. LMH0387 Connections for Equalizer-Only Mode
2
Pin
Name
Connection
2, 3, 43
VCCTX
GND
33, 34
SDI, SDI
GND
36
RREF
GND
40
TX_EN
GND
44
SD/HD
GND
45
TERMTX
GND
AN-2156 Configuring the LMH0387 for an Equalizer-Only Application
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SNLA130A – July 2011 – Revised April 2013
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LMH0387 Benefits over Dedicated Equalizer
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MOSI
SCK
47
46
RSVD
45
TERMTX
44
SD/HD
43
VCCTX
42
RSVD
41
VEE
40
TX_EN
39
MOSI
38
SCK
37
VEE
4
5
6
SDI IN
4.7 PF
7
8
9
VCCTX
SDI
SDI
VEE
RSVD
RSVD
RSVD
VEE
LMH0387
RSVD
VCCRX
BNC_IO
MISO
RSVD
SDO
SDO
1.0 PF
SS
VEE
36
35
34
33
32
31
VCC
30
29
28
27
26
EQUALIZER
OUT
25
DAP
SS
16
RSVD
RSVD
15
14
13
RSVD
10
RSVD
11
RSVD
12
RSVD
220:
RREF
VEE
EE
3
RSVD
VCCTX
RSVD
17 TERM
RX
18
SPI_EN
19
VEE
20
AEC+
21
AEC22
CD
23
CDTHRESH
24 V
1
2
RSVD
RSVD
48
MISO
VCC
CDTHRESH
CD
1.0 PF
Figure 1. LMH0387 Circuit for Equalizer-Only Mode
3
LMH0387 Benefits over Dedicated Equalizer
The LMH0387 configured as an equalizer provides several important benefits over a dedicated adaptive
cable equalizer (such as the LMH0384 or LMH0344), as described in the following sections.
3.1
Superior Return Loss
The LMH0387’s integrated return loss network provides superior return loss with no additional external
components required. SMPTE standards specify the requirements for the return loss of input and output
ports on video equipment (see Section 5). Return loss is essentially a measure of the impedance
matching, and is intended to prevent reflections.
This SMPTE requirement necessitates a return loss network consisting of inductors, resistors, and
capacitors to compensate for the input capacitance of the equalizer circuit. For good return loss, the
proper component choice and placement of this network are critical. By integrating this network inside the
package, the LMH0387 allows easy return loss compliance, with greater than 5 dB of margin above the
SMPTE specification.
Figure 2 shows the typical input return loss of the LMH0387, as measured on the SD387 evaluation board.
The LMH0387 input network on the SD387 evaluation board consists of an edge-launch BNC connector, a
4.7 uF AC-coupling capacitor, and a ¼-inch 75Ω PCB trace between the BNC and the LMH0387 (with a
similar layout to what is shown in Figure 4). Good return loss is achievable with the LMH0387 using a
variety of PCB layouts and BNC connectors (see Section 5).
SNLA130A – July 2011 – Revised April 2013
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AN-2156 Configuring the LMH0387 for an Equalizer-Only Application
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3
LMH0387 Benefits over Dedicated Equalizer
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0
RETURN LOSS (dB)
-5
SMPTE 424M Limit
-10
-15
-20
-25
-30
-35
-40
5
500
1000
1500
2000
2500
3000
FREQUENCY (MHz)
Figure 2. LMH0387 Input Return Loss
3.2
Reduced BOM Costs
The LMH0387 reduces bill of materials (BOM) costs and simplifies the PCB circuit required for equalizer
operation. The passive components required for return loss and the equalizer input termination are all
integrated within the package, greatly reducing the number external components.
Figure 3 shows the schematic of the input network typically required for a dedicated equalizer and the
corresponding schematic for the LMH0387. For the LMH0387, only a single capacitor is required between
the input pin and the BNC.
C1
C1
R1
C2
LMH03x4
Equalizer
LMH0387
(Equalizer Mode)
L1
R2
Figure 3. Input Circuit Required for Dedicated Equalizer (left) and LMH0387 (right)
3.3
Simplified Layout and PCB Space Savings
The LMH0387 greatly simplifies PCB layout while saving board space. For a dedicated equalizer, the
layout of the input network components is crucial for optimal operation and good return loss. This is not a
concern for the LMH0387 since this network is integrated. The LMH0387 layout is simple since all that is
required is the AC-coupling capacitor, and the placement of this capacitor is not critical.
While the LMH0387 device package is larger than the device package for a dedicated equalizer, the PCB
board area required for the LMH0387 is actually less than the area required for a dedicated equalizer such
as the LMH0384 or LMH0344, due to the additional external components necessary for the dedicate
equalizer. The LMH0387 can be placed closer to the BNC.
Figure 4 shows the PCB layout for the input network of a dedicated equalizer and the corresponding PCB
layout for the LMH0387.
4
AN-2156 Configuring the LMH0387 for an Equalizer-Only Application
Copyright © 2011–2013, Texas Instruments Incorporated
SNLA130A – July 2011 – Revised April 2013
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Conclusion
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R2
LMH03x4
Equalizer
R1
BNC
C1
L1
LMH0387
C2
SDI
BNC
C1
BNC_IO
Figure 4. Input Circuit PCB Layout for Dedicated Equalizer (left) and LMH0387 (right)
4
Conclusion
The LMH0387 configurable I/O can replace a dedicated adaptive cable equalizer in many SDI
applications. It is simple to configure the LMH0387 for equalizer-only use, and it provides several benefits,
including superior return loss, reduced BOM costs, easier PCB layout, and board space savings.
5
References
1. The SMPTE 424M, 292M, and 259M standards specify the requirements for the Serial Digital Interface
(SDI) used for transmission of digital video signals over a coaxial cable at 3G, HD, and SD data rates,
respectively. These standards specify the return loss requirements, and are listed below:
• SMPTE 424M-2006: 3 Gb/s Signal/Data Serial Interface
• SMPTE 292M-1998: Bit-Serial Digital Interface for High Definition Television Systems
• SMPTE 259M-2006: SDTV Digital Signal/Data – Serial Digital Interface
2. Return loss is dependent on board design. The following references discuss how to optimize PCB
board layout for SDI designs:
• Tsun-Kit Chin, Optimizing BNC PCB Footprint Designs for Digital Video Equipment
(videsignwire.com/optimizing-bnc-pcb-footprint-designs-for-digital-video-equipment/).
• Tsun-Kit Chin, High-Speed Board Layout Challenges in FPGA/SDI Sub-Systems (SNLA158).
3. LMH0387 3 Gbps HD/SD SDI Configurable I/O Adaptive Cable Equalizer / Cable Driver Data Sheet
(SNLS315).
SNLA130A – July 2011 – Revised April 2013
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AN-2156 Configuring the LMH0387 for an Equalizer-Only Application
Copyright © 2011–2013, Texas Instruments Incorporated
5
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