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 short-to-battery (STB) protection on the output, ideal for use in harsh infotainment environments ( For example, in the automotive field), CVBS video is transmitted. Although many video encoders (video DACs) such as the ADV7391 are capable of driving video loads directly, it is often beneficial to place a video driver at the output of the video encoder for power saving, filtering, line drive, and overvoltage circuitry. Protective function. The video driver is usually configured as an active filter (also called a reconstruction filter), whose main purpose is reflected in two aspects: to prevent the introduction of high-frequency components of the video signal (above the Nyquist frequency) during the sampling process; to provide gain to drive a 750Ω external cable connected to the video display.

Designers of infotainment and other video systems, such as rearview cameras and rear-seat entertainment systems, tend to use this type of circuitry to transmit video, for the reasons discussed above. However, there is a third pressing design issue, namely robustness. The ADA4432-1 and ADA4433-1 provide analog video designers with an integrated IC that provides critical overvoltage protection, enhanced ESD immunity, excellent video characteristics, lower power consumption, and line diagnostics Function.

The ADA4432-1 and ADA4433-1 are fully integrated video reconstruction filters, available in single-ended and differential types respectively. Both enable the output to have overvoltage protection (STB protection) up to 18 V, as well as low power consumption and line diagnostics. Line diagnostics are provided via logic outputs that can be activated in the event of a fault condition. The ADA4432-1 and ADA4433-1 include a high-order filter with a −3 dB cutoff frequency of 10 MHz and 45 dB rejection at 27 MHz.

With STB protection and robust ESD immunity, the ADA4432-1 and ADA4433-1 provide superior protection in harsh environments.

The ADV7391 and ADA4432-1 are fully automotive compliant and ideally suited for infotainment and visual safety systems in automotive applications. The ADV7391, ADA4432-1 and ADA4433-1 are available in very small LFCSP packages for small space applications.

 

The ADV7391 is a low-power, fully integrated digital video encoder that converts digital 8-bit component video data from CMOS imaging devices into standard analog baseband video signals that are compatible with global standards. Three 10-bit analog-to-analog video converters (operating with VAA = 2.6 V to 3.46 V) support standard definition (SD) or high definition (HD) video formats composite (CVBS), S-video (YC) or component (YPrPb/RGB) Analog output. The circuit in Figure 1 is configured to provide low output drive only through DAC1. To further reduce power consumption, other DACs and phase-locked loops (PLLs) are turned off. Low drive mode is defined as 4.33 mA full-scale output current. The ADV7391 contains an RSET pin. A resistor is connected between the RSET pin and AGND to control the full-scale output current. For low drive operation, RSET must equal 4.12kΩ and RL must equal 300Ω. The resistor connected to the RSET pin must have a 1% tolerance.

The ADV7391 contains an on-chip PLL that allows oversampling of video data. As shown in Figure 1, the PLL is disabled (subaddress 0x00, Bit 1 = 1), providing 2x the SD oversampling rate. With the PLL disabled, 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 unbalanced transmission lines. Pseudo-differential mode uses one conductor to carry the unbalanced data signal from the driver to the receiver, and the other conductor serves as the ground reference signal.

The positive conductor connects the ADA4432-1 output to the positive input of the differential receiver. The negative wire or ground conductor from the source circuit is connected to the negative input of the receiver. The output termination resistor of the ADA4432-1 should match the impedance of the receiver input termination resistor. For example, in a 75Ω system, each output of the ADA4432-1 is rear-terminated with 75Ω resistors, which are connected to a 75Ω resistor at the receiver.

In Figure 1, the ADA4432-1 is configured as a single-ended to single-ended driver, allowing unbalanced transmission using twisted pair, untwisted pair, or coaxial cable.

Low power considerations

Utilizing the ADA4432-1 or ADA4433-1 with series source termination and parallel load termination at low supply voltages can achieve significant power savings compared to driving the video cable directly from the DAC output . Figure 2 shows a video DAC driving the cable directly. A properly terminated DAC drive transmission line requires two 75 Ω loads in parallel, requiring more than 33mA to achieve a full-scale voltage level of 1.3 V. Figure 3 shows driving the same video load using the ADA4432-1 and series-parallel termination. This requires twice the output voltage to drive the 150Ω equivalent resistance, but only requires slightly more than 15 mA to achieve full-scale output. Using the same supply voltage as the DAC, this results in a power saving of 74% compared to the circuit in Figure 2. The high-order filtering provided by the ADA4432-1 reduces the DAC oversampling rate requirements, further reducing power consumption. The primary source of power savings in the configuration shown in Figure 3 is the low drive mode setting of the ADV7391. Combined with the reduction in oversampling requirements (PLL off) and required load current, this mode can significantly reduce power consumption.

See the ADV7391 data sheet for details on low drive mode.

 

 

EMI and EMC considerations

The analog output of a video DAC such as the ADV7391 requires low-pass filtering to remove unwanted signal components at frequencies above the sampling rate or frequency sidebands. Conversion of digital-to-analog signals creates repeated images in the frequency domain that are multiples of the sampling frequency. The main function of the reconstruction filter is to remove these frequency sideband components. Such filters significantly attenuate sideband signals, preventing aliasing when decoding the DAC output. Aliasing errors can cause image quality problems.

In addition, image frequency sidebands can cause radiation in output traces and circuits, potentially causing interference in adjacent circuits and other electronic systems. To reduce the impact of radiation, all unwanted high-frequency components should be removed before transmission along 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. Figures 4 to 6 illustrate this.

Figure 4 shows the spectrum of the CVBS video signal at the output of the ADV7391 without the ADA4432-1. The spectrum shows a signal with a content bandwidth of 6.5 MHz, with sidebands of 27 MHz, 54 MHz, 108 MHz, etc. The ADV7391 operates in full output drive mode with the PLL turned off and 2x oversampling.

 

Figure 5 shows the spectrum of the same CVBS signal at the output of the ADA4332-1 without the ADV7391. The difference here is that although the ADV7391 also operates in full output drive mode, the PLL is turned on and oversampled by 8x.

 

Figure 6 shows the spectrum of the same CVBS signal with the ADV7391 output filtered using the ADA4432-1. All sidebands are attenuated below 50 dB. The ADV7391 operates in low output drive mode with the PLL turned off and 2x oversampling.

 

PCB layout considerations

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

Decouple the ADV7391 power supply with 10µF and 0.1µF capacitors. Decouple the ADA4432-1 and ADA4433-1 output amplifiers with 0.1µF and 22µF capacitors for proper noise rejection and ripple reduction. These capacitors should be as close as possible to the corresponding device, and the 0.1μF capacitor should have a low ESR value. For all high frequency decoupling, ceramic capacitors are recommended.

It is important that the two ICs are as close to each other as possible. Power traces should be 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, please visit http://www.analog.com/CN0264-DesignSupport