A Robust Video Receiver with Input Short-to-Battery Protection
The ADA4830-1 is used to eliminate the common-mode noise and phase noise caused by the ground potential differences between an incoming video signal source and the receive circuit. More importantly, the ADA4830-1 and ADV7180 combination provides a very robust input that operates in the harsh automotive environment, and this combination provides protection from and detection of short-to-battery events and meets the strict requirements of automotive manufacturers.
This robust receiver circuit using the ADA4830-1 and ADV7180 follows the traditional, proven architecture of isolating/separating a low voltage integrated circuit like the ADV7180 from the outside world and using an amplifier circuit for signal conditioning and protection.
The ADA4830-1 (single) is a monolithic, high speed difference amplifier that integrates input overvoltage (short-to-battery) protection of up to 18 V with a wide input common-mode voltage range and excellent ESD robustness. It is intended for use as a receiver for differential or pseudo differential CVBS and other high speed video signals in harsh, noisy environments, such as automotive infotainment and vision systems. The ADA4830-1 combines high speed and precision, which allows for accurate reproduction of CVBS video signals, yet rejects unwanted common-mode error voltages.
The combination of STB protection/detection, robust ESD tolerance, and wide input common-mode voltage range allows the ADA4830-1 to be used as an automotive analog video receiver in systems such as rear-view cameras and rear seat entertainment.
The ADV7180 and ADA4830-1 are fully automotive qualified, which makes both products ideal for infotainment and vision-based safety systems for automotive applications. The ADV7180 and the ADA4830-1 are available in a very small LFCSP package which is ideal for space critical applications.
Figure 1. Robust Differential Video Receiver with the ADA4830-1 and the ADV7180 (All Connections and Decoupling Not Shown)
The short-to-battery protection that is integrated into the ADA4830-1 employs fast switching circuitry to clamp and hold internal voltage nodes at a safe level when an input overvoltage condition is detected. This protection allows the inputs of the ADA4830-1 to be directly connected to a remote video source, such as a rear-view camera, without the need for large expensive series capacitors.
Most video decoders, such as the ADV7180, are built on very low voltage processes and thus have a limited input voltage range. The ADA4830-1 has a signal gain of 0.5 V/V and is designed to keep the video signal within the allowed input range of the video decoder, which is typically 1 V p-p or less.
Input Common-Mode Voltage Range
In a standard, four resistor difference amplifier with 0.5 V/V gain, the input common-mode (CM) range is three times the CM range of the core amplifier. The input common-mode of the ADA4830-1 has been extended to more than ±8.5 V around ground (with a 5 V supply). This very wide common-mode range allows the ADA4830-1 and the ADV7180 to operate in the presence of very large common-mode offsets and noise without any adverse effects on image quality.
The ADA4830-1/ ADV7180 combination shown in Figure 1 offers a short-to-battery wire diagnostic by connecting the STB output on the ADA4830-1 to one of the GPIO ports of the ADV7180. During a short-to-battery event, the STB output is a logic low signal. The ADV7180 reads this low and generates an interrupt that can be read by a microcontroller in the system. The short-to-battery output flag (STB pin) is functionally independent of the short-to-battery protection. Its purpose is to indicate an overvoltage condition on either input. Because protection is provided passively, it is always available; the flag merely indicates the presence or absence of a fault condition.
Input ESD Protection
The protection architecture at the inputs of the ADA4830-1 uses a new technology for bidirectional asymmetrical blocking voltage. It is immune to short-to-battery conditions and able to provide ESD robustness above the 8 kV HBM level. For added ESD protection up to 15 kV, external transient suppressors are recommended.
Common-Mode Noise Rejection
The on-chip resistors integrated into the ADA4830-1 are inherently well matched, improving its common-mode rejection (CMR) performance over a wide frequency range. The CMR vs. frequency of the ADA4830-1 is shown in Figure 2 and is typically 65 dB at low frequencies, which enables the recovery of video signals in the presence of large levels of common-mode noise.
Figure 2. CMR vs. Frequency Response for Various Input Common-Mode Voltages
Common-mode errors, whether dc offsets or ac signals, degrade video image quality. Figure 3 and Figure 4 display a single large black stripe with a white background. Figure 3 shows the effects that a 500 kHz, 1 V p-p common-mode noise signal has on video image quality. Figure 4 shows the improved video image quality by adding the ADA4830-1 input stage to remove common-mode noise.
Figure 3. Video Display of a Black Stripe with 1 V p-p, 500 kHz Common-Mode Noise Inserted and ADA4830-1 Bypassed
Figure 4. Video Display of Black Stripe Showing 1 V p-p, 500 kHz Common-Mode Noise Rejected by the ADA4830-1
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 restore circuitry allow an input video signal peak-to-peak range of up to 1.0 V.
In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. The PCB should isolate the digital and analog sections as much as possible. The PCB was constructed in a 4-layer stack up with large area ground plane layers and power plane polygons. See the MT-031 Tutorial for more discussion on layout and grounding and the MT-101 Tutorial for information on decoupling techniques.
Decouple the power supply to the ADV7180 with 10 μF and 0.1 μF capacitors. In addition, decouple the ADA4830-1 with 0.1 μF and 22 μF capacitors to properly suppress noise and reduce ripple. Place the capacitors as close to the device as possible to ensure that the 0.1 μF capacitor has a low ESR value. Ceramic capacitors are recommended for all high frequency decoupling.
Ensure that power supply lines have as large a trace width as possible to provide low impedance paths and reduce glitch effects on the supply line. Shield clocks and other fast switching digital signals from other parts of the board by digital ground.
A complete design support package for this circuit note, including board layouts, complete schematic, and bill of materials, can be found at http://www.analog.com/CN0263-DesignSupport.
The following equipment is needed:
- A PC with a USB port and Windows® XP or Windows Vista® (32-bit), or Windows® 7 (32-bit).
- An Astrodesign VG-828 programmable video signal generator.
- A video source for signal and common-mode error voltage. For the test described in this circuit note, the AD8137 differential amplifier was used to convert single-ended video from the VG-828 to differential video, and the 500 kHz, 1 V p-p common-mode signal was applied to the VOCM input of the AD8137. The resulting differential signal with the added common-mode voltage was applied to the input of the evaluation board.
- A Hewlett-Packard 3314A function generator.
- An Agilent E3631A power supply.
- The EVAL-CN0263-EB1Z board.
- The CN-0263 evaluation software.
- A power supply: 7.5 V or 7.5 V wall wart.
- A video display for observing the analog video output of the EVAL-CN0263-EB1Z board.
Load the evaluation software by placing the CN-0263 evaluation software CD into the PC. Using My Computer, locate the drive that contains the evaluation software CD and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software.
Functional Block Diagram
See Figure 1 of this circuit note for the circuit block diagram and the EVAL-CN0263-EB1Z-SCH.pdf file for the circuit schematics. This file is located in the CN0263 Design Support Package.
With power to the supply off, connect a 7.5 V power supply to the +7.5 V and GND pins on the board. If available, a 7.5 V wall wart can be connected to the barrel connector on the board and used in place of the 7.5 V power supply. Connect the USB cable to the USB port on the PC. Do not connect the USB cable to the mini-USB connector on the board at this time.
Apply power to the 7.5 V supply (or wall wart) connected to the EVAL-CN0263-EB1Z circuit board. Launch the evaluation software and connect the USB cable from the PC to the mini-USB connector on the PCB.
Information and details regarding how to use the evaluation software for data capture can be found in the CN-0263 evaluation software Readme file.
|ADV7180||10-Bit, 4× Oversampling SDTV Video Decoder||
|ADA4830-1||High Speed Difference Amplifier with Input Short to Battery Protection||