AN-1240
APPLICATION NOTE
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Low Cost Video Multiplexer for Video Switching Using the ADA4853-2
Op Amp with Disable Function
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
This circuit provides a low cost, low power video multiplexer
using the ADA4853-2, a dual high speed amplifier. It allows the
user to input a fourth video source into a 3-channel video decoder
like the 40-lead LFCSP ADV7180. This saves both cost and
valuable board space by using a smaller, low cost decoder and
allows design reuse if only three video sources are required in a
similar design.
Table 1. Devices Connected/Referenced
Traditional CMOS multiplexers and switches suffer several
disadvantages at video frequencies where their on-resistance
introduces distortion, degrades differential gain and phase
performance, and interacts with the termination resistor to
attenuate the incoming video signal and affect the luminance.
System designers generally address these issues by adding
external buffers to add gain and increase drive capability.
Video multiplexing can be simplified by using high speed video
amplifiers with a disable/enable function (sometimes called
power-down). When the amplifier is disabled, its output stage
goes into a high impedance state. High speed video op amps
have all the key features required to make them ideal for this
function. Their high input impedance does not affect the
characteristic impedance of the transmission line, thus allowing
back termination. Because they are video amplifiers, they have
inherently good video specifications, including differential gain
and phase, slew rate, bandwidth, and 0.1 dB flatness.
For added benefit, each amplifier is configured as a four resistor
difference amplifier. This circuit eliminates some of the commonmode noise and phase noise caused by ground potential differences
between the incoming video signal and the circuit board ground.
Figure 1 only shows a single multiplexer circuit for simplicity,
but this circuit can be repeated for each input for as many times
as needed.
Product
ADA4853-2
ADV7180
Description
Low cost, high speed, dual, rail-to rail output
op amp
10-bit, 4× oversampling SDTV video decoder
The ADA4853-2 is a dual, low power, low cost, high speed, voltage
feedback, rail-to-rail output amplifier that has an operating
supply range of +2.65 V to +5 V. It is also available as a single
(ADA4853-1) or a triple (ADA4853-3). Operating on a single
5 V supply, the input multiplexer configuration shown in Figure 1
provides 14 MHz bandwidth (0.1 dB), gain of +2, and 58 dB offchannel isolation at 10 MHz. The 10 μs channel-to-channel
switching time supports CVBS analog video applications. In
Figure 1 the ADA4853-2 is configured as a low cost video 2:1
multiplexer for switching video signals.
The ADA4853-2 has individual disable pins for each amplifier.
Asserting any one of the disable pins to a voltage less than 1.2 V
will put the corresponding amplifier into a disabled state. In this
condition, the disabled amplifier presents a high output impedance
of approximately 100 kΩ at 5 MHz. The gain setting and feedback
resistors of the disabled amplifier appear as additional load to
the output of the active amplifier, but their values are very large
(approximately 2 kΩ) when compared to the 150 Ω video load
termination, and their effect is negligible.
The input stage of the ADA4853-2 includes protection from
large differential voltages that may be applied when disabled.
Internal clamp diodes limit this voltage to approximately ±1.4 V.
The high input-to-output OFF isolation of 55 dB at 10 MHz will
be maintained for voltages below this limit.
A critical design consideration that is important to note about
the four-resistor difference amplifier architecture is the effect of
resistor matching on the common-mode rejection (CMR) of
this circuit. The CMR can be calculated using the formula given
in Equation 1.
 1 + R2 


R1 
CMR = 20log10 
 4 Kr 


Rev. A | Page 1 of 3
(1)
AN-1240
Application Note
0.1µF
0.1µF
0.1µF
0.1µF
10nF
DISABLE1
AVDD _1.8V
DVDDIO
DVDD _1.8V
10nF
10nF
10nF
5V
AVDD _1.8V
PVDD _1.8V
75Ω
0.1µF
23
R1
R2
29
AIN2
DISABLE2
30
AIN3
AIN1
AIN2
20
36
14
27
P0
P1
P2
P3
P4
P5
P6
P7
AIN3
AVDD _1.8V
ADA4853
1/2 4
10
11
75Ω
26
0.1µF
22µF 1kΩ
1kΩ
R6
R5
P0
P1
P2
P3
P4
P5
P6
P7
YCrCb
8-BIT
656 DATA
VREFP
0.1µF
LLC
LOCATE CLOSE TO, AND
ON THE SAME SIDE AS,
THE ADV7180.
INTRQ
13
47pF
SFL
XTAL
VS/FIELD
28.63636MHz
1MΩ
HS
12
47pF
DVDDIO
POWER_DOWN
4kΩ
11
LLC
38
INTRQ
2
SFL
37
VS/FIELD
39
HS
XTAL1
PVDD _1.8V
32
18
ALSB
ELPF
19
10nF
PWRDWN
33
SCLK
SDATA
Figure 1. Low Cost Video Multiplexer Using the ADA4853-2 (Simplified Schematic)
Rev. A | Page 2 of 3
1.69kΩ
EXTERNAL
LOOP FILTER
08284-001
SDA
34
TEST_0
82nF
SCLK
40
3
15
35
C3
17
16
10
9
8
7
6
5
0.1µF
25
–
LFCSP–40
VREFN
22
R7
AGND
AGND
AGND
COMPOSITE
VIDEO IN 2
ADV7180BCPZ
9
C4
75Ω
RESET
13
28
21
24
+
RESET
R8
22µF 1kΩ
DGND
DGND
DGND
DGND
1kΩ
31
P[0:7]
PVDD
1kΩ
10nF
AVDD _1.8V
1
1
AVDD
C1
2
0.1µF
DVDD _1.8V
12
ADA4853
1/2
DVDD
–
14
3
DVDD
COMPOSITE
VIDEO IN 1
R3
C2
75Ω
22µF 1kΩ
DVDDIO _3.3V
10µF
4
22µF 1kΩ
0.1µF
DVDDIO
+
R4
DVDDIO
1kΩ
Application Note
AN-1240
In Equation 1, the term Kr is a single resistor tolerance in
fractional form (1% = 0.01, etc.), and it is assumed that the
amplifier has significantly higher CMR performance. Using this
formula, it shows that if 54 dB or better CMR is desired, then
R1, R2, R3, and R4 for amplifier 1 (and the corresponding
resistors for amplifier 2) need to have a matching tolerance of
0.1% or better. To prevent the incoming signal from violating
the allowable input and output voltage range, it is necessary to
provide a dc common-mode voltage. This is conveniently done
by connecting R4 and R8 to the 1.8 V analog supply voltage
(AVDD 1.8 V).
A function that is often overlooked, but very important, is
buffering. For example, many automotive customers use low
cost amplifiers to protect more expensive and complex devices,
such as video decoders and encoders. Amplifiers with ac- coupled
inputs, as shown in Figure 1, help protect such devices from
overvoltage and ESD damage.
The ADV7180 automatically detects and converts standard
analog baseband television signals compatible with worldwide
NSTC, PAL, and SECAM standards into 4:2:2 component video
data compatible with the 8-bit ITU-R.656 interface standards. The
accurate 10-bit analog-to-digital conversion provides professional
quality video performance for consumer applications with true
8-bit data resolution. Three analog video input channels accept
standard composite, S-video, or component video signals, supporting a wide range of consumer video sources. Automatic
gain control (AGC) and clamp restoration circuitry allow
processing of an input video signal with a peak-to-peak
range of up to 1.0 V.
LEARN MORE
Nisbett, Don. “Low-Cost Video Multiplexing Using High-Speed
Amplifiers.” Analog Dialogue. 43-01, Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-088 Tutorial, Analog Switches and Multiplexers Basics,
Analog Devices.
MT-089 Tutorial, Video Multiplexers and Crosspoint Switches,
Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Data Sheets
ADA4853-2 Data Sheet.
ADV7180 Data Sheet.
AD8170 Data Sheet.
AD8174 Data Sheet.
REVISION HISTORY
4/13—Rev. 0 to Rev. A
Changed Title of Document from CN-0076 to
AN-1240 .............................................................................. Universal
6/09—Revision 0: Initial Version
The circuit must be constructed on a multilayer PC board with
a large area ground plane. Proper layout, grounding, and
decoupling techniques must be used to achieve optimum
performance (see MT-031 Tutorial and MT-101 Tutorial).
COMMON VARIATIONS
If faster switching times and very high isolation are required,
then Analog Devices offers integrated multiplexers based on
complementary bipolar technology that offer a better solution
at video frequencies. The AD8170/AD8174/AD8180 and
AD8182 are bipolar video multiplexers that offer a high degree
of flexibility and are ideally suited for video applications.
Switching time for all devices in this family of products is 10 ns
to 0.1%. The off-channel isolation and crosstalk are typically
greater than 80 dB for this family of products.
©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08284-0-4/13(A)
Rev. A | Page 3 of 3