ML6421
LAYOUT CONSIDERATIONS
ML6421 VIDEO LOW PASS FILTER
In order to obtain full performance from these triple filters,
layout is very important. Good high frequency decoupling
is required between each power supply and ground.
Otherwise, oscillations and/or excessive crosstalk may
occur. A ground plane is recommended.
Filter Selection: The ML6421 provides several choices in
filter cut-off frequencies depending on the application.
RGB: When the BW of each signal is the same, then the
ML6421-1 (5.5MHz) or ML6421-3 (8MHz) are appropriate
depending on the sampling rate.
Each filter has its own supply and ground pins. In the test
circuit, 0.1µF capacitors are connected in parallel with
YUV: When the luminance bandwidth is different from the
color bandwidth, the ML6421-4 with the 8.0, and two
3.0MHz filters are most appropriate.
1nF capacitors on V , V C, V B and V A for
CC CC
CC
CC
maximum noise rejection (Figure 9).
Further noise reduction is achieved by using series ferrite
beads. In typical applications, this degree of bypassing
may not be necessary.
S-Video: For Y/C (S-video) and Y/C + CV (Composite
Video) systems the 5.5MHz or 8MHz filters are
appropriate. In NTSC the C signal occupies the bandwidth
from about 2.6MHz to about 4.6MHz, while in PAL the C
signal occupies the bandwidth from about 3.4MHz to
about 5.4MHz. In both cases, a 5.5MHz low pass filter
provides adequate rejection for both sampling and
reconstruction. In addition, using the same filter for both
Y/C and CV maintains identical signal timing without
adjustments.
Since there are three filters in one package, space the
signal leads away from each other as much as possible.
Power Considerations
The ML6421 power dissipation follows the formula:
2
ꢆ
!
"
ꢃ
$
ꢀ
ꢂV
ꢅ
OUT
Composite: When one or more composite signals need to
be filtered, then the 5.5MHz and 8MHz filters permit
filtering of one, two or three composite signals.
P = I × VCC
–
× 3
1CC
6
#
D
(1)
RL
ꢁ
ꢄ
#
This is a measure of the amount of current the part sinks
(current in – current out to the load).
NTSC/PAL: A 5.5MHz cut-off frequency provides good
filtering for 4.2MHz, 5.0MHz and 5.5MHz signals without
the need to change filters on a production basis.
Under worst case conditions:
2
ꢆ
!
"
× 3 = 872.5mW
ꢀ1.5 ꢃ
Sinx/x: For digital video system with output D/A
converters, there is a fall-off in response with frequency
P = 0.175 × 5.5 –
0
5
#
D
ꢂ 75 ꢅ
ꢁ ꢄ
#
$
4
THEORETICAL SINX/X
CORRECTION FOR
13.5MHz SAMPLING
+5V
ML6421
DIGITAL
INPUTS
ANALOG
OUTPUTS
RED DAC
(CURRENT SOURCING
R
8
R
G
B
2
0
75Ω
75Ω
75Ω
GREEN DAC
(CURRENT SOURCING
G
8
BLUE DAC
(CURRENT SOURCING
B
8
–2
–4
DAC LOAD
ADJUSTED FOR
2V
SINX/X ERROR FOR
TYPICAL DAC AT 13.5MHz
P-P
0
1
2
3
4
5
6
7
FREQUENCY (MHz)
Figure 11. Typical ML6421 Reconstruction Application
Figure 10. Sinx/x Frequency Response
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
