TMS320DM8148_1109 Datasheet, PDF (269/360 Pages) Texas Instruments – TMS320DM814x DaVinci Digital Media Processors

TMS320DM8148, TMS320DM8147, TMS320DM8146
www.ti.com
SPRS647B – MARCH 2011 – REVISED SEPTEMBER 2011
8.10.2 Video DAC Guidelines and Electrical Data/Timing
The device's analog video DAC outputs 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.
TV_OUTx
R_OUT
Reconstruction
Filter^(A)
~9.5 MHz
C_AC^(B)
TV_VFBx
A. Reconstruction Filter (optional)
B. AC coupling capacitor (optional)
Figure 8-41. TV Output (Normal Mode)
TV_VFBx
R_LOAD
Reconstruction
Filter^(A)
~9.5 MHz
Amplifier
3.7 V/V
75 Ω
C_AC^(B)
A. Reconstruction Filter (optional). Note: An amplifier with an integrated reconstruction filter can alternatively be used
instead of a discrete reconstruction filter.
B. 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 R_LOAD 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 R_OUT 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 (i.e., Y and C for S-Video output
should match each other).
Copyright © 2011, Texas Instruments Incorporated
Peripheral Information and Timings 269
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