Circuit Note
CN-0060
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
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ADA4851-1
Low Cost, High Speed, Rail-to-Rail Output
Amplifier
ADV7180
10-Bit, 4× Oversampling SDTV Video
Decoder
Low Cost Differential Video Receiver Using the ADA4851 Amplifier
and the ADV7180 Video Decoder
CIRCUIT FUNCTION AND BENEFITS
digitized by the ADV7180 video decoder. This circuit eliminates
common-mode noise and phase noise caused by ground
potential difference from an incoming video signal and is well
suited for automotive infotainment and vision safety systems.
The circuit described in this document and shown in Figure 1
provides a low cost, low power, unipolar, differential receiver
using the ADA4851-1 for an incoming video signal before being
DVDD _1.8V
0.1µF
DVDDIO
0.1µF
0.1µF
10nF
10nF
DVDD _1.8V
AIN3
R1
29
30
31
RESET
26
AIN2
20
14
4
36
27
PVDD
AIN1
P0
P1
P2
P3
P4
P5
P6
P7
AIN3
RESET
ADV7180BCPZ
VREFN
LFCSP–40
0.1µF
LLC
0.1µF
25
VREFP
INTRQ
SFL
LOCATE CLOSE TO, AND ON THE
SAME SIDE AS, THE ADV7180.
VS/FIELD
13
47pF
XTAL
HS
17
16
10
9
8
7
6
5
11
38
2
37
39
1MΩ
POWER_DOWN
SCLK
LLC
INTRQ
SFL
VS/FIELD
HS
PVDD _1.8V
18
34
33
19
10nF
1.69kΩ
82nF
ALSB
PWRDWN
SCLK
SDATA
40
3
15
35
SDA
32
ELPF
AGND
AGND
AGND
4kΩ
XTAL1
28
21
24
12
DVDDIO
YCrCb
8-BIT
656 DATA
EXTERNAL LOOP FILTER
DGND
DGND
DGND
DGND
28.63636MHz
47pF
P0
P1
P2
P3
P4
P5
P6
P7
TEST_0
C1 R2
22µF 1kΩ
AIN2
P[0:7]
Figure 1. Low Cost Differential Video Receiver Using the ADA4851-1(Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. C | Page 1 of 3
08626-001
R6
1kΩ
2
RF
1kΩ
–
23
10nF
22
4
1
0.1µF
AVDD
ADA4851
75Ω
R5
1kΩ
DVDD
COMPOSITE
VIDEO
IN
6
DVDD
3
DVDDIO
1
C2 R3
22µF 1kΩ
+
0.1µF
AVDD _1.8V
10µF
DVDDIO
0.1µF
+
10nF
PVDD _1.8V
DVDDIO _3.3V
R4
1kΩ
0.1µF
10nF
5V
AVDD _1.8V
AVDD _1.8V
CN-0060
Circuit Note
The ADA4851 family of amplifiers along with the ADV7180
video decoder are all automotive qualified which makes both
products ideal for auto-vision applications. Low power, low
cost, high speed, and fast settling make these amplifiers well
suited for many video applications where these requirements
are very important. Figure 1 shows only a single amplifier for
simplicity; the amplifier circuit can be repeated for each input
as many times as needed.
CIRCUIT DESCRIPTION
The ADA4851-1 is a single (also available as a dual or quad
amplifier in a single package), 130 MHz, low power, low cost,
high speed, voltage feedback, rail-to-rail output amplifier that
has an operating supply range of +3 V to +5 V. In Figure 1, the
ADA4851-1 is configured as a low cost differential-to-singleended receiver for video signals.
This configuration is a standard four-resistor difference amplifier
optimized for composite video frequencies, with a differentialto-single-ended gain of 1. The amplifier only amplifies the
difference between the two inputs, while eliminating the
common-mode noise between the incoming signals and
allowing the reconstruction of the original signal. Using low
value, high accuracy resistors and a high CMRR amplifier
provides the exceptional performance.
A critical design consideration that is important to note about
this architecture is its dependence on resistor matching for the
common-mode rejection (CMR) of this circuit design. The
CMR can be calculated using the following formula:
R2 

1+

R1 
CMR = 20 Log10 
 4 KR 




where KR is a single resistor tolerance in fractional form (for
example, 1% = 0.01). It is assumed that the amplifier has
significantly higher CMR performance. This formula shows
that if 54 dB or better CMR is desired, then R1, R2, R3, and R4
must have a matching tolerance of 0.1% or better.
To prevent the incoming signal from violating the amplifier
input and output voltage range, it is necessary to add a dc offset.
This is conveniently done by connecting R4 to the 1.8 V analog
supply voltage (AVDD_1.8V) also used by the ADV7180, which
helps keep the cost low. To prevent supply noise from coupling
into the video signal stream, be sure to have adequate bypassing
on this supply (AVDD_1.8V), as shown in Figure 1.
Some automotive applications require ac-coupled inputs to
protect the input to the amplifier from a short to a voltage
higher than its supply voltage. The input impedance of 1 kΩ
(R2 and R3) with the 22 μF capacitors (C1 and C2) sets a high-
pass corner frequency pole at approximately 7 Hz. If a lower
frequency is desired, increasing the capacitor value decreases the
corner frequency proportionally. For example, using 47 μF
capacitors results in a 3.4 Hz high-pass corner frequency.
R5 and R6 form a typical voltage divider network that is
required to keep the input video signal within the allowed range
of the ADC, 0 V to 1 V. This circuit should be placed before all
analog inputs to the ADV7180. If the amplitude of the analog
video signal is too high, clipping may occur, resulting in visual
artifacts. The voltage divider impedance of can be scaled to help
reduce the output power of the ADA4851. For high impedance
dividers, care must be taken to keep the ADA4851 and the
ADV7180 very close together, on the PCB, to reduce the trace
parasitic capacitance.
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. AGC and clamp-restore circuitry allow an input video
signal peak-to-peak range of up to 1.0 V.
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
There are a few options for supplying the dc bias voltage for the
ADA4851-1. The ADR12x family of voltage references has a
wide range of values that can be used. Another method is to add
one more resistor to form a voltage divider in conjunction with
R4. Be sure that the parallel combination of R4 and the new R5
matches the value of the RF feedback resistor.
If better CMR performance at high frequencies and higher
input impedance are desired, the ADA4851-1 can be replaced
with the AD8130 differential line receiver.
Rev. C | Page 2 of 3
Circuit Note
CN-0060
LEARN MORE
REVISION HISTORY
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
6/11—Rev. B to Rev. C
MT-061 Tutorial, Instrumentation Amplifier (In-Amp) Basics,
Analog Devices.
2/11—Rev. A to Rev. B
MT-068 Tutorial, Difference and Current Sense Amplifiers,
Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Data Sheets and Evaluation Boards
ADA4851-1 Data Sheet.
Changes to Circuit Function and Benefits.....................................2
Changes to Figure 1 .......................................................................... 1
Changes to Circuit Description....................................................... 2
11/09—Rev. 0 to Rev. A
Updated Format ................................................................. Universal
2/09—Revision 0: Initial Release
ADV7180 Data Sheet.
AD8130 Data Sheet.
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CN08626-0-6/11(C)
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