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ML6428CS2

S-Video Filter and 75 Line Drivers with Summed Composite Output

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ML6428

DATA SHEET

Functional Description

The ML6428 is a dual monolithic continuous time video

filter designed for reconstructing the luminance and

chrominance signals from an S-Video D/A source.

Composite video output is generated by summing the Y

and C outputs. The ML6428CS-1 is intended for use in AC

coupled input and output applications. The ML6428CS-2 is

intended for AC coupled input and DC coupled output

applications (see Figures 5 and 6).

The filters have a 4th-order Butterworth characteristic with

an optimization toward low overshoot and flat group delay.

All outputs are capable of driving 2VP-P into 150Ω video

loads, with up to 35pF of load capacitance at the output pin.

ML6428CS-1 outputs are AC coupled, ML6428CS-2 outputs

are DC coupled. The CV output can drive two video loads

plus a high-impedance modulator. Thus the CV output is

intended to simultaneously drive a VCR, a TV, and a high-

impedance modulator. Y and C are capable of driving a 75Ω

load at 1VP-P. The ML6428 is capable of driving two com-

posite loads and a TV modulator simultaneously.

All channels are clamped during sync to establish the appro-

priate output voltage swing range. Thus the input coupling

capacitors do not behave according to the conventional RC

time constant. Clamping for all channels settles within 2ms

of a change in video input sources.

In most applications, the ML6428's input coupling capacitors

are 0.1µF. The Y input sinks 1.6µA during active video,

which nominally tilts a horizontal line by 2mV (max) at

the Y output (Figure 4). During sync, the clamp typically

sources 20µA to restore the DC level. The net result is that

the average input current is zero.

Any change in the input coupling capacitor's value will

inversely alter the amount of tilt per line. Such a change

will also linearly affect the clamp response times.

The C channel has no pulldown current sources and is essen-

tially tilt-free. Its input is clamped by a feedback amp which

responds to the CV output. Since CV = Y+C, the CV output

will droop by the same amount as Y during active video, and

will rise by the same amount as Y during sync.

The ML6428 is robust and stable under all stated load and

input conditions. Capacitavely bypassing both VCC pins

directly to ground ensures this performance. (See Figures 5

and 6)

Luminance (Y) I/O

The luma input is driven by either a low impedance source of

1VP-P or the output of a 75Ω terminated line. The input is

required to be AC coupled via a 0.1uF coupling capacitor

which allows for a nominal settling time of 2ms. The luma

output is capable of driving a 150Ω load at 2VP-P or 1VP-P

into a 75Ω load. ML6428CS-1 outputs are AC coupled,

ML6428CS-2 outputs are DC coupled.Up to 35pF of load

capacitance (at the output pin) can be driven without stability

or slew issues. A 220µF AC coupling capacitor is recom-

mended at the output (ML6428-1 only).

Chrominance (C) I/O

The chroma input is driven by a low impedance source of

0.7VP-P or the output of a 75Ω terminated line. The input is

required to be AC coupled via a 0.1uF coupling capacitor

which allows for a nominal clamping time of 1ms. The

chroma output is capable of driving a 150Ω load at 2VP-P

or 1VP-P into a 75Ω load. ML6428CS-1 outputs are AC

coupled, ML6428CS-2 outputs are DC coupled. Up to 35pF

of load capacitance can be driven without stability or slew

issues. A 220µF AC coupling capacitor is recommended at

the output (ML6428-1 only).

Composite video (CV) output

The composite video output is capable of driving 2 CV loads

to 2VP-P and a high input impedance CV modulator.

ML6428CS-1 outputs are AC coupled, ML6428CS-2 outputs

are DC coupled. It is intended to drive three devices: TV,

VCR, and a modulator. The TV or VCR input can be shorted

to ground and the other outputs will still meet specifications.

Up to 35pF of load capacitance (at the output pin) can be

driven without stability or slew issues.

Using the ML6428 for PAL Applications

The ML6428 can be optimized for PAL video by adding

frequency peaking to the composite and S-video outputs.

Figures 7 and 8 illustrate the use of a additional external

capacitor, 330pF, added in parallel to the output source

termination resistor. This raises the frequency response from

1.6 dB down at 4.8Mhz to 0.35dB down at 4.8MHz allowing

for accurate reproduction of the upper sideband of the PAL

subcarrier. Figure 9 shows the frequency response of PAL

video with various values of peaking capacitors (0pF, 220pF,

270pF, 330pF) between 0 and 10MHz.

For NTSC applications without the peaking capacitor the

rejection at 27MHz is 42dB (typical) while for PAL applica-

tions with the peaking capacitor the rejection at 27MHz is

38dB (typical). This is shown in Figure 10. The differential

group delay is shown in Figure 11 with and without a peak-

ing capacitor (0pF, 220pF, 270pF, and 330pF) varies slightly

with capacitance, going from 8ns to 13ns.

4

REV. 1B April 2003

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