ADV7177KS View Datasheet(PDF) - Analog Devices

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ADV7177KS

Integrated Digital CCIR-601 to PAL/NTSC Video Encoder

Analog Devices 

APPENDIX 6

OPTIONAL DAC BUFFERING

ADV7177/ADV7178

For external buffering of the ADV7177/ADV7178 DAC out-

puts, the configuration in Figure 67 is recommended. This

configuration shows the DAC outputs running at half (18 mA)

their full current (34.7 mA) capability. This will allow the

ADV7177/ADV7178 to dissipate less power, the analog current is

reduced by 50% with a RSET of 300 Ω and a RLOAD of 75 Ω. This

mode is recommended for 3.3 volt operation as optimum perfor-

mance is obtained from the DAC outputs at 18 mA with a VAA of

3.3 volts. This buffer also adds extra isolation on the video out-

puts, see buffer circuit in Figure 68. When calculating absolute

output full current and voltage, use the following equation:

VOUT = IOUT × RLOAD

( ) IOUT = VREF × K

RSET

K = 4.2146 constant , VREF = 1.235 V

VAA

ADV7177/ADV7178

VREF

DAC A

PIXEL

PORT

DIGITAL

CORE

300⍀

RSET

DAC B

DAC C

OUTPUT

BUFFER

OUTPUT

BUFFER

OUTPUT

BUFFER

75⍀

75⍀

75⍀

Figure 67. Output DAC Buffering Configuration

INPUT

VCC

36⍀

2N2907

75⍀

OUTPUT TO

TV/MONITOR

75⍀

Figure 68. Recommended Output DAC Buffer

REV. 0

–37–

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