ML6423CS-2 datasheet(7/10 Pages) MICRO-LINEAR

7 / 10 page

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 VCC 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:

P(I

)

DCC

OUT

$##

This is a measure of the amount of current the part sinks

(current in – current out to the load).

Under worst case conditions:

PmW

D =

$##

. )

0175 5 5

8725

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.

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.

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 75

W or

150

W). Resistance values should be adjusted to provide

1VP-P at the input of the ML6423. The ML6423 will drive

W source termination resistors (making the total load

150

W) so that no external drivers or amplifiers are

required.

Figure 5a. Sinx/x Frequency Response

–2

–4

FREQUENCY (MHz)

01234567

THEORETICAL SINX/X

CORRECTION FOR

13.5MHz SAMPLING

SINX/X ERROR FOR

TYPICAL DAC AT 13.5MHz

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