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AD722JR-16-REEL 데이터시트(PDF) 9 Page - Analog Devices |
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AD722JR-16-REEL 데이터시트(HTML) 9 Page - Analog Devices |
9 / 12 page AD722 REV. 0 –9– resistor is required close to each AD722 output, while 75 Ω to ground should terminate the far end of each line. The outputs have a dc bias and must be ac coupled for proper operation. The COMP and LUMA outputs have information down to 30 Hz that must be transmitted. Each output requires a 220 µF series capacitor to work with the 75 Ω resistance to pass these low frequencies. The CRMA signal has information mostly up at the chroma frequency and can use a smaller ca- pacitor if desired, but 220 µF can be used to minimize the num- ber of different components used in the design. Displaying VGA Output on a TV The AD722 can be used to convert the analog RGB output from a personal computer’s VGA card to the NTSC or PAL television standards. To accomplish this it is important to understand that the AD722 requires interlaced RGB video and clock rates that are consistent with those required by the television standards. In most computers the default output is a noninterlaced RGB sig- nal at a frame rate higher than used by either NTSC or PAL. Most VGA controllers support a wide variety of output modes that are controlled by altering the contents of internal registers. It is best to consult with the VGA controller manufacturer to determine the exact configuration required to provide an inter- laced output at 60 Hz (50 Hz for PAL). Basically these two standards use most of the features of the standard that their names imply, but use the subcarrier that is equal to or approximately equal to the frequency of the other standard. Because of the automatic programming of the filters in the chrominance path and other timing considerations, it is not possible to support these standards. Layout Considerations The AD722 is an all CMOS mixed signal part. It has separate pins for the analog and digital +5 V and ground power supplies. Both the analog and digital ground pins should be tied to the ground plane by a short, low inductance path. Each power supply pin should be bypassed to ground by a low inductance 0.1 µF capacitor and a larger tantalum capacitor of about 10 µF. The three analog inputs (RIN, GIN, BIN) should be terminated with 75 Ω to ground close to the respective pins. However, as these are high impedance inputs, they can be in a loop-thru configuration. This technique is used to drive two or more devices with high frequency signals that are separated by some distance. A connection is made to the AD722 with no local termination, and the signals are run to another distant device where the termination for these signals is provided. The output amplitudes of the AD722 are double that required by the devices that it drives. This compensates for the halving of the signal levels by the required terminations. A 75 Ω series Figure 15. Interfacing the AD722 to the (Interlaced) VGA Port of a PC +5V (VAA) 0.1µF 75 Ω 649 Ω 649 Ω 75 Ω 649 Ω 649 Ω 75 Ω 649 Ω 649 Ω AD813 5 6 7 11 12 13 14 9 10 8 1/3 1/3 –5V 1 2 3 4 13 2 FIN OSC 0.1µF +5V ** * CRYSTAL AGND DGND 14 15 7 9 10 11 1 4 5 AD722 ENCD RIN GIN BIN HSYNC VSYNC SELECT STND CRMA LUMA CMPS APOS DPOS + 10–30pF 0.1µF 10µF 0.1µF 10µF 75 Ω 75 Ω JMP * PARALLEL–RESONANT CRYSTAL; 3.579545MHz (NTSC) OR 4.433620MHz (PAL) CAPACITOR VALUE DEPENDS ON CRYSTAL CHOSEN **FSC OR 4FSC CLOCK; 3.579545MHz, 14.31818MHz (NTSC) OR 4.433620MHZ, 17.734480MHz(PAL) + 75 Ω 220µF COMPOSITE VIDEO + 75 Ω 220µF + 75 Ω 220µF Y C S-VIDEO (Y/C VIDEO) 75 Ω 75 Ω 75 Ω B G R RGB MONITOR VSYNC HSYNC FROM VGA PORT +5V +5V 10k Ω +5V 10k Ω SELECT 75 Ω 1/3 JMP 12 3 16 8 6 6 7 8 9 10 11 12 13 14 15 + +5V 1 2 3 4 5 VGA OUTPUT CONNECTOR |
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