The circuit in Figure 1 shows a digital-to-analog video converter paired with a low cost, low power, fully integrated reconstruction video filter with output short-to-battery (STB) protection, ideal for CVBS video transmission in harsh infotainment environments such as automotive applications. Although many video encoders (video DACs), such as the ADV7391, can drive a video load directly, it is often beneficial to use a video driver at the output of a video encoder for power savings, filtering, line driving, and overvoltage circuit protection. The main purpose of a video driver, typically configured as an active filter (also known as a reconstruction filter), is twofold: it blocks the higher frequency components (above the Nyquist frequency) that were introduced into the video signal as part of the sampling process, and it provides gain to drive the external 75 Ω cable to the video display.
Designers of infotainment and other video systems, such as rearview cameras and rear-seat entertainment systems, are likely to use this circuit to transmit video for the reasons previously stated. However, a third pressing design issue centers on the robustness. The ADA4432-1 and ADA4433-1 provide analog video designers with integrated ICs that offer crucial overvoltage protection, hardened ESD tolerance, along with excellent video specification, low power consumption, and wire diagnostic features.
The ADA4432-1 and ADA4433-1 are fully integrated, single-ended and differential video reconstruction filters, respectively. They combine overvoltage protection (STB protection) up to 18 V on the outputs, with low power consumption and a wire diagnostic capability. Wire diagnostics are provided by way of a logic output that is activated when a fault condition is present. The ADA4432-1 and ADA4433-1 feature a high-order filter with a −3 dB cutoff frequency of 10 MHz and 45 dB of rejection at 27 MHz.
The ADV7391, and ADA4432-1 are fully automotive qualified, which makes both products ideal for infotainment and visionbased safety systems for automotive applications. The ADV7391, ADA4432-1, and the ADA4433-1 are available in a very small LFCSP package ideal for small footprint applications.
The ADV7391 is a low power, fully integrated digital video encoder that converts digital 8-bit component video data from a CMOS imager into a standard analog baseband video signal compatible with worldwide standards. Three, 10-bit digital-to-analog video converters (operating on VAA = 2.6 V to 3.46 V) provide support for composite (CVBS), S-video (YC), or component (YPrPb/RGB) analog outputs in either standard definition (SD) or high definition (HD) video formats. The circuit in Figure 1 is configured for low output drive through DAC1 only. To conserve more power, the other DACs and phase-locked loop (PLL) are turned off. Low drive mode is defined as 4.33 mA full-scale output current. The ADV7391 contains an RSET pin. A resistor connected between the RSET pin and AGND is used to control the full-scale output current. For low drive operation, RSET must equal 4.12 kΩ, and RL must equal 300 Ω. The resistor connected to the RSET pin must have a 1% tolerance.
The ADV7391 includes an on-chip, PLL that allows for over-sampling video data. As shown in Figure 1, the PLL is disabled (Subaddress 0x00, Bit 1 = 1) providing an SD oversample rate of 2×. With the PLL disabled, the external loop filter components are removed to save space and cost.
The ADA4432-1 can be used as a pseudo differential (single-ended) driver with an unbalanced transmission line. The pseudo differential mode uses a single conductor to carry an unbalanced data signal from the driver to the receiver, while a second conductor is used as a ground reference signal.
The positive conductor connects the ADA4432-1 output to the positive input of a differential receiver. The negative wire or ground conductor from the source circuitry connects to the negative input of the receiver. The output termination of the ADA4432-1 should match the impedance of the input termination at the receiver. For example, in a 75 Ω system, each output of the ADA4432-1 is back terminated with 75 Ω resistors that are connected to a resistance of 75 Ω at the receiver.
In Figure 1, the ADA4432-1 is configured as a single-ended-to-single-ended driver that allows unbalanced transmission using twisted pair cable, untwisted cable, or coaxial cable.
Low Power Considerations
Using a series source termination and a shunt load termination on a low supply voltage with the ADA4432-1 or the ADA4433-1 realizes significant power savings compared to driving a video cable directly from a DAC output. Figure 2 shows a video DAC driving a cable directly. Properly terminated, a DAC driven transmission line requires two 75 Ω loads in parallel, demanding in excess of 33 mA to reach a full-scale voltage level of 1.3 V. Figure 3 shows the same video load being driven using the ADA4432-1 and a series-shunt termination. This requires two times the output voltage to drive the equivalent of 150 Ω but only requires a little more than 15 mA to reach a full-scale output. When running on the same supply voltage as the DAC, this results in a 74% reduction in power consumption compared to the circuit in Figure 2. The high-order filtering provided by the ADA4432-1 lowers the requirements on the DAC oversampling ratio, thereby realizing further power savings. The main source for power savings realized by the configuration shown in Figure 3 comes from the low drive mode setting for the ADV7391. This along with the reduction in the requirement for oversampling (PLL turned off) and the reduced load current required results in significant power savings.
For more information on low drive mode, refer to the ADV7391 data sheet.
Figure 2. Driving a Video Transmission Line Directly with a DAC
EMI and EMC Considerations
The analog output of video DACs like the ADV7391 requires low-pass filtering to remove unwanted signal components at frequencies more than the sample rate or frequency sidebands. The conversion of a digital-to-analog signal creates duplicated images in the frequency domain, at multiples of the sampling frequency. Removing these frequency sideband components is the main function of the reconstruction filters. These filters significantly attenuate the sideband signals, preventing aliasing when the DAC outputs are decoded. Aliasing error can create image quality issues.
In addition, image frequency sidebands can create radiation emissions in the output traces and cabling that are potentially disruptive to adjacent circuitry and other electronic systems. To reduce the effect of radiation emissions, remove all unwanted high frequency components before transmitting along the printed circuit board (PCB) traces and transmission cables. The ADA4432-1 helps reduce EMI by filtering the DAC output and removing unwanted high frequency content. Figure 4 to Figure 6 illustrate this point.
Figure 4 shows the frequency spectrum of a CVBS video signal at the output of the ADV7391 without the ADA4432-1. The spectrum shows a signal whose content bandwidth is 6.5 MHz, with sidebands at 27 MHz, 54 MHz, 108 MHz, and so on. The ADV7391 is operating in full output drive mode with the PLL turned off at 2× oversampling.
Figure 4. CVBS Measured Directly at the Output of the ADV7391, PLL Off, 2× Oversampling, Full Output Drive Mode
Figure 5 show the frequency spectrum of the same CVBS signal at the output of the ADV7391 without the ADA4332-1. The difference here is that the ADV7391 is operating in full output drive mode with the PLL turned on at 8× oversampling.
Figure 5. CVBS Measured Directly at the Output of the ADV7391, PLL On, 8× Oversampling, Full Output Drive Mode
Figure 6 shows the frequency spectrum of the same CVBS signal with the ADA4432-1 filtering the output of the ADV7391. All sidebands are attenuated to less than 50 dB. The ADV7391 is operating in low output drive mode with the PLL turned off at 2× oversampling.
Figure 6. CVBS Measured at the Output of the ADA4432-1, PLL Off, 2× Oversampling, Low Output Drive Mode
PCB Layout Considerations In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. Isolate the digital and analog sections of the PCB as much as possible. This 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 ADV7391 with 10 μF and 0.1 μF capacitors. Decouple the ADA4432-1 and the ADA4433-1 output amplifiers 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 with the 0.1 μF capacitor having a low ESR value. Ceramic capacitors are advised for all high frequency decoupling.
It is important to keep the two ICs as close to each other as possible. Power supply lines should 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 the board layouts, can be found at http://www.analog.com/CN0264-DesignSupport.
The ADA4433-1 is a fully differential filter/driver that can be used as a single-ended-to-differential amplifier or as a differential-todifferential amplifier. In Figure 7, the ADA4433-1 is configured as a single-ended-to-differential output driver. In single-endedto- differential output applications, bias the INN input appropriately to optimize the output range. To make the most efficient use of the output range of the ADA4433-1, especially with low supply voltages, it is important to allow the differential output voltage to swing in both a positive and negative direction around the output common-mode voltage (VOCM) level; the midsupply point (1.65 V).
To do this, the −IN input is biased at the midpoint of the expected input signal range, as shown in Figure 7. This is done with a voltage divider to the supply voltage (7.5 kΩ and 1.33 kΩ connected between the 3.3 V supply and GND biases −IN to 0.5 V). The 0.1 μF capacitor helps to filter high frequency supply noise. A 1 V p-p single-ended signal on +IN, with −IN biased at 0.5 V produces a differential input voltage of −0.5 V to +0.5 V. The resulting differential output swings above and below the VOCM level (1.65 V). The ADA4433-1 output voltage now extends from 1.15 V to 2.15 V, requiring only 1 V of the output range to produce a 1 V p-p signal at the receiver.
The differential outputs of the ADA4433-1 allow fully balanced transmission using twisted or untwisted pair cable. In this configuration, the differential output termination consists of one source resistor on each output. Both resistors are equal to half the receiver input termination. For example, in a 75 Ω system, each output of the ADA4433-1 is back terminated with 37.5 Ω resistors connected to a differential resistance of 75 Ω at the receiver.
This circuit uses the EVAL-CN0264-EB1Z circuit board, which contains the circuit to be evaluated, as described in this note. The Cypress USB microcontroller is used to configure and load software to and from the EVAL-CN0264-EB1Z board.
Equipment Needed The following equipment is needed:
Load the evaluation software by placing the CN0264 evaluation CD in the CD drive of 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-CN0264-EB1Z-SCH.pdf file for the circuit schematics. This file is contained in the CN0264 Design Support Package.
With power to the supply off, connect a 7.5 V power supply to the 7.5 V terminal and the GND terminal 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- CN0264-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-0264 evaluation software Readme file.