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Circuit Note CN-0060
Circuit Note CN-0060 Devices Connected/Referenced Circuit Designs Using Analog Devices Products Apply these product pairings quickly and with confidence. For more information and/or support call 1-800-AnalogD (1-800-262-5643) or visit www.analog.com/circuits. 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 www.BDTIC.com/ADI 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 www.BDTIC.com/ADI 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. (Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "Circuits from the Lab". Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any "Circuits from the Lab" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners. ©2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08626-0-6/11(C) Rev. C | Page 3 of 3 www.BDTIC.com/ADI