LT6207IGN datasheet(13/18 Pages) LINER

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LT6205/LT6206/LT6207

620567fc

APPLICATIONS INFORMATION

black intensity, thereby causing scan retrace activity to be

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

(or set up 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

differential-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-ter-

minated 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.,

150Ω). 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 data sheet, 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 ampliﬁer

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 additional 1.12V

allowance must be made in the ampliﬁer supply (assum-

ing gain of 2, so VMIN = 3.12 + VOH +VOL). For example,

an LT6205 operating on 5V has a conservative 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 outputs 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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