LT6207IGN datasheet(11/16 Pages) LINER

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LT6205/LT6206/LT6207

620567f

APPLICATIO S I FOR ATIO

invisible on a CRT. The “black” level of the waveform is at

(or “setup” very slightly above) the upper limit of the sync

information. Waveform content above the black-level is

intensity information, with peak brightness represented at

the maximum signal level. In the case of composite video,

the modulated color subcarrier is superimposed on the

waveform, but the dynamics remain inside the 1VP-P limit

(a notable exception is the chroma ramp used for differen-

tial-gain and differential-phase measurements, which can

reach 1.15VP-P).

DC-Coupled Video Amplifier Considerations

Typically video amplifiers drive cables that are series

terminated (“back-terminated”) at the source and load-

terminated at the destination with resistances equal to the

cable characteristic impedance, Z0 (usually 75Ω). This

configuration forms a 2:1 resistor divider in the cabling

that must be accounted for in the driver amplifier by

delivering 2VP-P output into an effective 2 • Z0 load (e.g.

150

Ω). Driving the cable can require more than 13mA

while the output is approaching the saturation-limits of the

amplifier output. The absolute minimum supply is: VMIN =

2 + VOH +VOL. For example, the LT6206 dual operating on

3.3V as shown on the front page of this datasheet, with

exceptionally low VOH ≤ 0.5V and VOL ≤ 0.35V, provides a

design margin of 0.45V. The design margin must be large

enough to include supply variations and DC bias accuracy

for the DC-coupled video input.

Handling AC-Coupled Video Signals

AC-coupled video inputs are intrinsically more difficult to

handle than those with DC-coupling because the average

signal voltage of the video waveform is effected by the

picture content, meaning that the black-level at the ampli-

fier “wanders” with scene brightness. The wander is

measured as 0.56V for a 1VP-P NTSC waveform changing

from black-field to white-field and vice-versa, so an addi-

tional 1.12V allowance must be made in the amplifier

supply (assuming gain of 2, so VMIN = 3.12 + VOH +VOL).

For example, an LT6205 operating on 5V has a conserva-

tive design margin of 1.03V. The amplifier output (for gain

of 2) must swing +1.47V to –1.65V around the DC-

operating point, so the biasing circuitry needs to be

designed accordingly for optimal fidelity.

Clamped AC-Input Cable Driver

A popular method of further minimizing supply require-

ments with AC-coupling is to employ a simple clamping

scheme as shown in Figure 2. In this circuit, the LT6205

operates from 3.3V by having the sync-tips control the

charge on the coupling capacitor C1, thereby reducing the

black-level input wander to

≈ 0.07V. The only minor

drawback to this circuit is the slight sync-tip compression

(

≈ 0.025V at input) due to the diode conduction current,

though the picture content remains full fidelity. This circuit

has nearly the design margin of its DC-coupled counter-

part, at 0.31V (for this circuit, VMIN = 2.14 + VOH +VOL). The

clamp-diode anode bias is selected to set the sync-tip

output voltage at or slightly above VOL.

YPbPr to RGB Component-Video Converter

The back-page application uses the LT6207 quad to imple-

ment a minimum amplifier count topology to transcode

consumer component-video into RGB. In this circuit,

signals only pass through one active stage from any input

to any output, with passive additions being performed by

the cable back-termination resistors. The compromise in

using passive output addition is that the amplifier outputs

must be twice as large as that of a conventional cable

driver. The Y-channel section also has the demanding

requirement that it single-handedly drives all three out-

puts to full brightness during times of white content, so a

helper current source is used to assure unclipped video

when operating from

±5V supplies. This circuit maps

sync-on-Y to sync on all the RGB channels, and for best

results should have input black-levels at 0V nominal to

prevent clipping.

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