ADV7172 datasheet(51/59 Pages) AD

51 / 59 page

ADV7172/ADV7173

–51–

REV. A

If an output filter is required for the CVBS, Y, UV, Chroma

and RGB outputs of the ADV7172/ADV7173, the filter shown

below can be used. The plot of the filter characteristics is

shown in Figure 93. An Output Filter is not required if the

outputs of the ADV7172/ADV7173 are connected to most

analog monitors or analog TVs; however, if the output signals

are applied to a system where sampling is used (e.g., Digital

TVs), then a filter is required to prevent aliasing.

APPENDIX 7

OPTIONAL OUTPUT FILTER

2.2 H

22pF

270pF

330pF

FILTER I/P

FILTER O/P

Figure 92. Output Filter Used with Output Buffer

FREQUENCY – Hz

10k

100M

100k

10M

–5

–70

–10

–15

–20

–25

–30

–35

–40

–45

–50

–55

–60

–65

VdB – OP

Figure 93. Output Filter Plot

APPENDIX 8

OPTIONAL DAC BUFFERING

When external buffering is needed of the ADV7172/ADV7173

DAC outputs, the configuration in Figure 94 is recommended.

This configuration shows the DAC outputs, A, B, C, running at

half (18 mA) their full current (36 mA) capability. This will

allow the ADV7172/ADV7173 to dissipate less power; the analog

current is reduced by 50% with a RSET1 = 300

Ω and R

SET2 =

600

Ω and an R

LOAD of 75

Ω. This mode is recommended for

3.3 V operation as optimum performance is obtained from the

DAC outputs at 18 mA with a VAA of 3.3 V. This buffer also

adds extra isolation on the video outputs (see buffer circuit in

Figure 95). Note that DACs D, E and F will always require

buffering as the full-scale output current from these DACs is

limited to 8.66 mA. With DACs A, B and C, buffering is op-

tional, based on the user requirements for performance and power

consumption.

When calculating absolute output full-scale current and voltage,

use the following equations:

VOUT

= I

OUT

× R

LOAD

IOUT

VREF

× K

()

RSET

= 4.2146 constant ,V

REF

= 1.235 V

ADV7172/ADV7173

VREF

PIXEL

PORT

VAA

OUTPUT

BUFFER

DAC A

CVBS

CHROMA

LUMA

300

RSET1

OUTPUT

BUFFER

DAC B

OUTPUT

BUFFER

DAC C

OUTPUT

BUFFER

DAC D

OUTPUT

BUFFER

DAC E

OUTPUT

BUFFER

DAC F

DIGITAL

CORE

600

RSET2

Figure 94. Output DAC Buffering Configuration

300

OUTPUT TO

OUTPUT FILTER/

TV MONITOR

INPUT

+VCC

AD847

–VCC

Figure 95. Recommended Output DAC Buffer

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