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ON Semiconductor and the ON Semiconductor logo are trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries. ON Semiconductor owns the rights to a number
of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf. ON Semiconductor reserves the right
to make changes without further notice to any products herein. ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products and applications using ON
Semiconductor products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON
Semiconductor data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s
technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not designed, intended, or authorized for use as a critical component in life support systems or any FDA
Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended
or unauthorized application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out
of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor
is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
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AN-6040
Video and Cable Driving Fundamentals
Summary
Any transmission path; such as transmission lines,
long trace lengths (utilizing strip line or micro strip
techniques), and cables, must be properly terminated
to maintain optimum signal quality. Also, most highspeed amplifiers are not designed to remain stable
when driving large capacitive loads. This application
note explains the need for proper termination
techniques and methods for driving capacitive or
reactive loads.
Rs
KM4100
CL
Rf
Rg
Figure 1.
Driving a Capacitive Load
or Reactive Load
Driving a coaxial cable without using a series resistor
can cause frequency peaking or oscillation. Figure 3
illustrates a typical circuit configuration for driving
coaxial cable. The resistors Rs and RL are equal to the
characteristic impedance, Zo, of the cable or
transmission line. The amplifier's output impedance
increases with increased frequency. The capacitor, C,
can be used to match the cable over a greater
frequency range: it compensates for the amplifier's
increasing output impedance.
Typical Topology for Driving a Capacitive Load
0.8
0.6
0.4
Amplitude (V)
Driving a capacitive load directly reduces the phase
margin of an amplifier. The capacitive load and the
amplifier's output impedance cause phase lag, which
results in an under-damped pulse response or
oscillation. Some amplifiers are capable of directly
driving large capacitive loads, but others require a
series resistance to buffer the output stage. Refer to
the amplifier datasheet to determine to which
category the amplifier belongs. A small series
resistance (Rs) at the output of the amplifier,
illustrated in Figure 1, improves stability and settling
performance. Figure 2 shows the resulting pulse
responses for a high-speed amplifier driving a 100pF
capacitive load with and without series resistance.
0.2
Rs=20_CL=100pF
Rs=0_CL=100pF
0
-0.2
-0.4
-0.6
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
TIME (us)
Figure 2.
Pulse Response with and without RS
C
Zo
Rs
RL
Rf
Rg
Figure 3.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/27/06
RL
Driving Cable or Transmission Line
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AN-6040
APPLICATION NOTE
Driving Video Loads
When driving a video load, usually a 75Ω coaxial cable, it
is important to utilize a doubly terminated configuration,
similar to Figure 4. To ensure maximum stability and
performance, it is important to use both a source termination
resistor and an end termination resistor. When a 75Ω cable
Figure 4.
is used, make both termination resistors 75Ω. The
termination resistors attenuate the signal by a factor of 2 or
6dB. Set the amplifier’s gain to counter this effect. If a 1Vpp
signal is applied to the input and 1Vpp is desired at output of
the cable, set the amplifiers gain to 6dB or 2V/V.
One Channel of FHP3xx0 Driving Two Video Loads, Illustrating Doubly Terminated Configuration
Figure 5 shows a pulse response at various probe points of a
circuit using only source termination. The noticeable “blip”
in the pulse response is the reflection caused by the seven
feet of cable. Figure 6 shows only end termination with the
Figure 5.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/27/06
same “blip” visible. With proper source and end termination,
a clean pulse response is received, as shown in Figure 7.
Source Termination
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2
AN-6040
APPLICATION NOTE
Figure 6.
Figure 7.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/27/06
End Termination
Proper Source and End Termination
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3
AN-6040
APPLICATION NOTE
Driving Multiple Video Loads
Both the FHP3x30 and FHP3x50 family of amplifiers
offer ample output current for driving long cable lengths
or multiple video loads. The FHP3230 can easily drive
four video loads, but having enough drive capability is
not the only concern. As an amplifier drives multiple
video loads, its differential gain and phase contribution
increase. The usefulness of the extremely low differential
Figure 8.
gain and phase of the FHP3x30 family becomes evident
as it drives more loads. Figure 8 shows the resulting
differential gain and phase (less than 0.2% and 0.2°) of
the FHP3230 driving four video loads or 32.5Ω.
Differential Gain / Phase Driving Four Video Loads
Summary
A few simple steps ensure maximum stability and highspeed performance driving capacitive or reactive loads.
ƒ
Related Datasheets
FHP3130, FHP3230, FHP3430 — Single, Dual, and Quad,
High Speed, 2.7V to 12V, Rail-to-Rail Amplifiers
http://www.fairchildsemi.com/ds/FH/FHP3230.pdf
When driving a capacitive load, review the amplifier
datasheet to determine if a series resistance is needed.
Fairchild’s amplifier datasheets provide a plot showing
the recommended value for a given capacitive load.
ƒ
Use proper (matched) termination resistors to ensure
optimum signal performance and prevent reflections.
ƒ
When using a doubly terminated load, double the
amplifier gain to maintain the desired voltage signal
levels at the output.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/27/06
FHP3350, FHP3450 — Triple and Quad Voltage Feedback
Amplifiers
http://www.fairchildsemi.com/ds/FH%2FFHP3350.pdf
FHP3194 — 4:1 High-Speed Multiplexer
http://www.fairchildsemi.com/ds/FH%2FFHP3194.pdf
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4
AN-6040
APPLICATION NOTE
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS
HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE
APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS
PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION.
As used herein:
1.
Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, or (c) whose failure to perform
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to
result in significant injury to the user.
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/27/06
2.
A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
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ON Semiconductor and
are trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries.
ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent
coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards,
regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or
specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer
application by customer’s technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not
designed, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification
in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized
application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and
expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such
claim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This
literature is subject to all applicable copyright laws and is not for resale in any manner.
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