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
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