ML6423
APPLICATION GUIDELINES
(Continued)
LAYOUT CONSIDERATIONS
In order to obtain full performance from these dual 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.
Each filter has its own supply and ground pins. In the test
circuit, 0.1µF capacitors are connected in parallel with
1nF capacitors on all V
CC
pins for maximum noise
rejection (Figure 4).
Further noise reduction is achieved by using series ferrite
beads. In typical applications, this degree of bypassing
may not be necessary.
Since there are two filters and a sum output driver in one
package, space the signal leads away from each other as
much as possible.
POWER CONSIDERATIONS
The ML6423 power dissipation follows the formula:
In a typical application (Figure 5b) the ML6423 is used as
the final output device in a video processing chain. In this
case, inputs to the ML6423 are supplied by DAC outputs
with their associated load resistors (typically 75W or
150W). Resistance values should be adjusted to provide
1V
P-P
at the input of the ML6423. The ML6423 will drive
75W source termination resistors (making the total load
150W) so that no external drivers or amplifiers are
required.
Composite:
When one or more composite signals need to
be filtered, then the 5.5MHz and 9.6MHz filters permit
filtering of one, two, or three composite signals.
Over Sampling:
While the ML6423 filters can eliminate
the need for over sampling combined with digital
filtering, there are times when over sampling is used. For
these situations, 9.3MHz could be used in place of
5.5MHz.
NTSC/PAL:
A 5.5MHz cutoff frequency provides good
filtering for 4.2MHz, 5.0MHz and 5.5MHz signals
without the need to change filters on a production basis.
Sinx/x:
For digital video system with output D/A
converters, there is a fall off in response with frequency
due to discrete sampling. The fall off follows a sinx/x
response (Figure 5a). The ML6423 filters have a
complementary boost to provide a flatter overall
response. The boost is designed for 13.5MHz Y/C and CV
sampling and 6.75MHz U/V sampling.
P
D
=
(I
CC
V
CC
V
)
-
!
RL
OUT
2
3
"#
#$
This is a measure of the amount of current the part sinks
(current in – current out to the load).
Under worst case conditions:
P
D
15
3
"#
=
8725mW
.
=
(0.175
5.5)
-
75
$#
.
!
2
4
THEORETICAL SINX/X
CORRECTION FOR
13.5MHz SAMPLING
2
FILTER SELECTION
The ML6423 provides several choices in filter cutoff
frequencies depending on the application.
S-Video:
For Y/C (S-video) and Y/C + CV (Composite
Video) systems the 5.5MHz or 9.6MHz 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
lowpass 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.
AMPLITUDE
0
–2
SINX/X ERROR FOR
TYPICAL DAC AT 13.5MHz
–4
0
1
2
3
4
5
FREQUENCY (MHz)
6
7
Figure 5a. Sinx/x Frequency Response
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
