ADV7170/ADV7171
APPENDIX 7—OPTIONAL OUTPUT FILTER
APPENDIX 8—OPTIONAL DAC BUFFERING
If an output filter is required for the CVBS, Y, UV, Chroma, and
RGB outputs of the ADV7170/ADV7171, the filter shown in
Figure 79 can be used. Plots of the filter characteristics are
shown in Figure 80. An output filter is not required if the
outputs of the ADV7170/ADV7171 are connected to most
analog monitors or analog TVs. However, if the output signals
are applied to a system where sampling is used (for example,
digital TVs), then a filter is required to prevent aliasing.
When external buffering of the ADV7170/ADV7171 DAC
outputs is needed, the configuration in Figure 81 is recom-
mended. This configuration shows the DAC outputs running
at half (18 mA) their full current (36 mA) capability. This allows
the ADV7170/ADV7171 to dissipate less power; the analog
current is reduced by 50ꢀ with a RSET of 300 Ω and a RLOAD of
75 Ω. This mode is recommended for 3.3 V operation, because
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 the buffer circuit in Figure 82).
22pF
When calculating absolute output full-scale current and voltage,
use the following equations:
1.8μH
FILTER I/P
FILTER O/P
270pF
330pF
75R
VOUT = IOUT × RLOAD
VREF × K
RSET
)
IOUT
=
Figure 79. Output Filter
K = 4.2146 constant,VREF =1.235V
0
10
20
30
40
50
60
70
V
AA
ADV7170/ADV7171
OUTPUT
BUFFER
V
REF
DAC A
DAC B
DAC C
DAC D
CVBS
OUTPUT
BUFFER
CVBS
PIXEL
PORT
DIGITAL
CORE
OUTPUT
BUFFER
LUMA
R
SET
OUTPUT
BUFFER
300Ω
CHROMA
80
100k
1M
10M
FREQUENCY (Hz)
100M
Figure 80. Output Filter Plot
Figure 81. Output DAC Buffering Configuration
V
+
CC
5
4
3
OUTPUT TO
TV MONITOR
1
AD8061
INPUT/
OPTIONAL
FILTER O/P
2
V
–
CC
Figure 82. Recommended Output DAC Buffer
Rev. C | Page 48 of 64
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