AM3874 datasheet(265/361 Pages) TI

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LOAD

TV_VFBx

Reconstruction

Filter

~9.5 MHz

(A)

Amplifier

3.7 V/V

(B)

75 Ω

OUT

TV_OUTx

TV_VFBx

Reconstruction

Filter

~9.5 MHz

(A)

(B)

AM3874, AM3871

www.ti.com

SPRS695C – SEPTEMBER 2011 – REVISED DECEMBER 2013

8.10.2 Video DAC Guidelines and Electrical Data/Timing

The analog video DAC outputs of the device can be operated in one of two modes: Normal mode and

TVOUT Bypass mode. In Normal mode, the device’s internal video amplifier is used. In TVOUT Bypass

mode, the internal video amplifier is bypassed and an external amplifier is required.

Figure 8-41 shows a typical circuit that permits connecting the analog video output from the device to

standard 75-

Ω impedance video systems in Normal mode. Figure 8-42 shows a typical circuit that permits

connecting the analog video output from the device to standard 75-

Ω impedance video systems in TVOUT

Bypass mode.

Reconstruction Filter (optional)

AC coupling capacitor (optional)

Figure 8-41. TV Output (Normal Mode)

Reconstruction Filter (optional). Note: An amplifier with an integrated reconstruction filter can alternatively be used

instead of a discrete reconstruction filter.

AC coupling capacitor (optional)

Figure 8-42. TV Output (TVOUT Bypass Mode)

During board design, the onboard traces and parasitics must be matched for the channel. The video DAC

output pins (TV_OUTx/TV_VFBx) are very high-frequency analog signals and must be routed with

extreme care. As a result, the paths of these signals must be as short as possible, and as isolated as

possible from other interfering signals. In TVOUT Bypass mode, the load resistor and amplifier/buffer

should be placed as close as possible to the TV_VFBx pins. Other layout guidelines include:

•

Take special care to bypass the VDDA_VDAC_1P8 power supply pin with a capacitor. For more

information, see Section 7.2.9, Power-Supply Decoupling.

•

In TVOUT Bypass mode, place the RLOAD resistor as close as possible to the Reconstruction Filter

and Amplifier. In addition, place the 75-

Ω resistor as close as possible (< 0.5 ") to the Amplifier/buffer

output pin. To maintain a high-quality video signal, the onboard traces after the 75-

Ω resistor should

have a characteristic impedance of 75

Ω (± 20%).

•

In Normal mode, TV_VFBx is the most sensitive pin in the TV out system. The ROUT resistor should

be placed as close as possible to the device pins. To maintain a high-quality video signal, the onboard

traces leading to the TV_OUTx pin should have a characteristic impedance of 75

Ω (± 20%) starting

from the closest possible place to the device pin output.

•

Minimize input trace lengths to the device to reduce parasitic capacitance.

•

Include solid ground return paths.

•

Match trace lengths as close as possible within a video format group (that is, Y and C for S-Video

output should match each other).

Peripheral Information and Timings

265

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