The circuit shown in Figure 1 provides an extremely robust solution with integrated overvoltage (short to battery [STB]) protection for receiving CVBS video signals in harsh environments. It uses the ADA4830-1 , a low-cost, low-power, unipolar, differential receiver that converts fully differential or pseudo-differential (ground-referenced single-ended) video signals to single-ended signals before being digitized by the ADV7180 .

The ADA4830-1 is used to eliminate common-mode noise and phase noise, which is caused by ground potential differences between the input video signal source and the receiving circuit. More importantly, the combination of the ADA4830-1 and ADV7180 provides very robust input characteristics that can operate in harsh automotive environments, can also protect and detect battery short-circuit events, and comply with the strict requirements of automobile manufacturers.

This robust receiver circuit uses the ADA4830-1 and ADV7180 and conforms to traditional, proven isolated/stand-alone low-voltage integrated circuit architectures such as the ADV7180 which comes from the outside and uses amplifier circuitry for signal conditioning and protection.

The ADA4830-1 (single channel) is a monolithic high-speed differential amplifier with integrated input overvoltage (battery short circuit) protection up to 18V, providing a wide input common-mode voltage range and excellent ESD robustness. It is designed as a receiver for differential or pseudo-differential CVBS and other high-speed video signals, and is suitable for working in harsh high-noise environments, such as automotive infotainment and vision systems.

The ADA4830-1 combines high speed and precision to accurately reproduce CVBS video signals while rejecting unwanted common-mode error voltages. With STB protection/detection, robust ESD tolerance, and a wide input common-mode voltage range, the ADA4830-1 can be used as an automotive analog video receiver in systems such as rearview cameras and rear-seat entertainment.

The ADV7180 and ADA4830-1 are fully automotive compliant and ideally suited for infotainment and visual safety systems in automotive applications. The ADA4830-1 is available in a very small LFCSP package for space-constrained applications.

The ADA4830-1 is a monolithic high-speed differential amplifier designed for automotive applications. The design is based on a traditional 4-resistor difference amplifier and is optimized to avoid common mistakes made when enhancing circuits for this standard amplifier application.

When an input overvoltage condition is detected, the short-to-battery protection feature integrated into the ADA4830-1 utilizes fast switching circuitry to clamp and maintain the internal voltage node at a safe level. This protection allows the input of the ADA4830-1 to be connected directly to a remote video source, such as a rearview camera, without the need for expensive series bulk capacitors.

Most video decoders like the ADV7180 are built for extremely low voltage processing and therefore have a limited input voltage range. The ADA4830-1 has a signal gain of 0.5V/V and is designed to keep the video signal within the allowed input range of the video decoder, typically 1Vp-p or less.

Input common mode voltage range

In a standard 4-resistor differential amplifier with a gain of 0.5V/V, the input common-mode (CM) range is three times the CM range of the core amplifier. The input common mode of the ADA4830-1 is extended to ±8.5V above ground (5 V supply). This extremely wide common-mode voltage range allows the ADA4830-1 and ADV7180 to operate with large common-mode offsets and noise without any adverse impact on image quality.

Line diagnostics

The ADA4830-1/ADV7180 combination shown in Figure 1 has short-to-battery circuit diagnostics by connecting the STB output on the ADA4830-1 to one of the ADV7180's GPIO ports. During a short-to-battery event, the STB output is a logic low signal. The ADV7180 reads this low level and generates an interrupt that can be read by the system microcontroller. The battery short circuit output flag (STB pin) function has nothing to do with battery short circuit protection. Its purpose is to indicate the overvoltage status of each output. Since the protection is provided passively, it is always effective; the flag only indicates whether a fault is currently occurring.

Input ESD protection

The architecture that protects the inputs of the ADA4830-1 uses a new technology of bidirectional asymmetric blocking voltage. It is not affected by battery short circuits and has ESD robustness exceeding an 8 kVHBM rating. For additional ESD protection up to 15kV, an external transient suppressor is recommended.

Common mode noise rejection

The ADA4830-1's integrated on-chip resistors are well matched internally, adding common-mode rejection (CMR) performance over a wide frequency range. Figure 2 shows the relationship between ADA4830-1 CMR and frequency. The typical value at low frequency is 65dB, which can restore the video signal when the common mode noise is large.

Common-mode errors, whether DC bias or AC signals, degrade video image quality. Figures 3 and 4 show a single giant black stripe on a white background. Figure 3 shows the impact of a 500 kHz, 1Vp-p common mode noise signal on video image quality. Figure 4 shows the enhanced video image quality with the addition of the ADA4830-1 input stage to remove common-mode noise.

The ADV7180 automatically detects standard analog baseband television signals compatible with global NSTC, PAL and SECAM standards and converts them into 4:2:2 component video data compatible with the 8-bit ITU-R.656 interface standard. Targeted at consumer electronics applications with true 8-bit data resolution, precise 10-bit analog-to-digital conversion delivers professional-quality video performance. Three analog video input channels accept standard composite, S-Video or component video signals, supporting a wide range of consumer video sources. The automatic gain control (AGC) and clamp reset circuit enable the peak-to-peak range of the input video signal to reach 1.0 V.

Printed circuit board (PCB) layout considerations

In any circuit where precision is important, power and ground return layout on the circuit board must be carefully considered. The PCB should isolate the digital and analog parts as much as possible. This PCB is made of 4-layer boards stacked with large area polygons for the ground layer and power layer. See the MT-031 guide for a detailed discussion of layout and grounding, and the MT-101 guide for information on decoupling techniques .

Decouple the ADV7180 power supply with 10µF and 0.1µF capacitors. In addition, the ADA4830-1 is decoupled with 0.1μF and 22μF capacitors for proper noise suppression and ripple reduction. These capacitors should be placed as close as possible to the corresponding device to ensure a low ESR value for the 0.1µF capacitor. For all high frequency decoupling, ceramic capacitors are recommended.

Make sure the power traces are as wide as possible to provide a low impedance path and reduce the effects of glitches on the power lines. Clocks and other fast-switching digital signals are digitally shielded so that they do not affect other components on the circuit board.

For a complete design support package for this circuit note, including board layout, complete schematic and bill of materials, see http://www.analog.com/CN0263-DesignSupport