Noty an57fa Linear Technology

Video Circuit Collection

Jon Munson and Frank Cox

INTRODUCTION

Application Note 57

January 1994

Even in a time of rapidly advancing digital image process­ing, analog video signal processing still remains eminently
viable. The video A/D converters need a supply of properly
amplifi ed, limited, DC restored, clamped, clipped, con­toured, multiplexed, faded and fi ltered analog video before
they can accomplish anything. After the digital magic is
performed, there is usually more amplifying and fi ltering
to do as an adjunct to the D/A conversion process, not to

CIRCUIT INDEX

I. Video Amplifi er Selection Guide ..................................................................................... 2

II. Video Cable Drivers .................................................................................................... 3

AC-Coupled Video Drivers ............................................................................................................................. 3

DC-Coupled Video Drivers ............................................................................................................................. 4

Clamped AC-Input Video Cable Driver ........................................................................................................... 5

Twisted-Pair Video Driver and Receiver ......................................................................................................... 5

III. Video Processing Circuits ............................................................................................. 6

ADC Driver .................................................................................................................................................... 6

Video Fader ................................................................................................................................................... 7

Color Matrix Conversion ................................................................................................................................ 7

Video Inversion ........................................................................................................................................... 10

Graphics Overlay Adder ............................................................................................................................... 10

Variable Gain Amplifi er ................................................................................................................................ 12

Black Clamp ................................................................................................................................................ 12

Video Limiter ............................................................................................................................................... 13

Circuit for Gamma Correction ...................................................................................................................... 14

LT1228 Sync Summer ................................................................................................................................. 16

mention all those pesky cables to drive. The analog way
is often the most expedient and effi cient, and you don’t
have to write all that code.

The foregoing is only partly in jest. The experienced engineer
will use whatever method will properly get the job done;
analog, digital or magic (more realistically, a combination
of all three). Presented here is a collection of analog video
circuits that have proven themselves useful.

IV. Multiplexer Circuits .................................................................................................. 17

Integrated Three-Channel Output Multiplexer .............................................................................................. 17

Integrated Three-Channel Input Multiplexer ................................................................................................ 18

Forming RGB Multiplexers from Triple Amplifi ers ....................................................................................... 20

Stepped Gain Amplifi er Using the LT1204 ................................................................................................... 21

LT1204 Amplifi er/Multiplexer Sends Video Over Long Twisted Pair ............................................................ 21

Fast Differential Multiplexer ......................................................................................................................... 22

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

an57fa

AN57-1

Application Note 57

V. Misapplications of CFAs ............................................................................................. 23

VI. Appendices –– Video Circuits from Linear Technology Magazine ............................................ 24

A. Temperature-Compensated, Voltage-Controlled Gain Amplifi er Using the LT1228 .................................. 24

B. Optimizing a Video Gain-Control Stage Using the LT1228 ....................................................................... 26

C. Using a Fast Analog Multiplexer to Switch Video Signals for NTSC “Picture-in-Picture” Displays .......... 30

Video Amplifi er Selection Guide

PART GBW (MHz) CONFIGURATION COMMENTS

LT6553 1200 (A = 2) T A = 2 (Fixed), 6ns Settling Time

LT6555 1200 (A = 2) T 2:1 MUX, A = 2 (Fixed)

LT1226 1000 (A

LT6557 1000 (A = 2) T A = 2 (Fixed), Automatic Bias for Single Supply

LT6554 650 (A = 1) T A = 1 (Fixed), 6ns Settling Time

LT6556 650 (A = 1) T 2:1 MUX, A = 1 (Fixed)

LT1222 500 (A

LT1395/LT1396/LT1397 400 S, D, Q CFA, DG = 0.02%, DP = 0.04%, 0.1dB Flat to 100MHz

LT1818/LT1819 400 S, D 900V/µs SR, DG = 0.07%, DP = 0.02%

LT1192 350 (A

LT1194 350 (A

LT6559 300 T CFA, Independant Enable Controls, Low Cost

LT1398/LT1399 300 D, T CFA, Independant Enable Controls

LT1675-1/LT1675 250 (A = 2) S, T 2:1 MUX, A = 2 (Fixed)

LT1815/LT1816/LT1817 220 S, D, Q 750V/µs SR, DG = 0.08%, DP = 0.04%

LT6210/LT6211 200 S, D CFA, Adjustable Speed and Power

LT1809/LT1810 180 S, D Low Voltage, Rail-to-Rail Input and Output

LT1203/LT1205 170 D, Q MUX, 25ns Switching, DG = 0.02%, DP = 0.04°

LT1193 160 (A

LT1221 150 (A

LT1227 140 S CFA, 1100V/µs SR, DG = 0.01%, DP = 0.01°, Shutdown

LT1259/LT1260 130 D, T RGB CFA, 0.1dB Flat to 30MHz, DG = 0.016%, DP = 0.075°, Shutdown

LT6550/LT6551 110 (A = 2) T, Q Low Voltage, Single Supply, A = 2 (Fixed)

LT1223 100 S CFA, 12-Bit Accurate, Shutdown, 1300V/µs SR, Good DC Specs, DG = 0.02%, DP = 0.12°

LT1229/LT1230 100 D, Q CFA, 1000V/µs SR, DG = 0.04%, DP = 0.1°

LT1252 100 S CFA, DG = 0.01%, DP = 0.09°, Low Cost

LT1812 100 S Low Power, 200V/µs SR

LT6205/LT6206/LT6207 100 S, D, Q 3V Single Supply

LT1191 90 S Low Voltage, ±50mA Output

LT1253/LT1254 90 D, Q CFA, DG = 0.03%, DP = 0.28°, Flat to 30MHz, 0.1dB

LT1813/LT1814 85 D, Q Low Power, 200V/µs SR

LT1228 80 (gm = 0.25) S Transconductance Amp + CFA, Extremely Versatile

LT6552 75 (A

LT1204 70 S CFA, 4-Input Video MUX Amp, 1000V/µs SR, Superior Isolation

≥ 25) S 400V/µs SR, Good DC Specs

V

≥ 10) S 12-Bit Accurate

V

≥ 5) S Low Voltage, ±50mA Output

V

= 10) S Differential Input, Low Voltage, Fixed Gain of 10

V

≥ 2) S Low Voltage, Differential Input, Adjustable Gain, ±50mA Output

V

≥ 4) S 250V/µs SR, 12-Bit Accurate

V

≥ 2) S Differential Input, Low Power, Low Voltage

V

an57fa

AN57-2

Application Note 57

PART GBW (MHz) CONFIGURATION COMMENTS

LT1363/LT1364/LT1365 70 S, D, Q 1000V/µs SR, I

LT1206 60 S 250mA Output Current CFA, 600V/µs SR, Shutdown

LT1187 50 (A

≥ 2) S Differential Input, Low Power

V

LT1190 50 S Low Voltage

LT1360/LT1361/LT1362 50 S, D, Q 600V/µs SR, I

LT1208/LT1209 45 D, Q 400V/µs SR

LT1220 45 S 250V/µs, Good DC Specs, 12-Bit Accurate

LT1224 45 S 400V/µs SR

LT1189 35 (A

≥ 10) S Differential Input, Low Power, Decompensated

V

LT1995 32 (A = 1) S Internal Resistor Array

LT1213/LT1214 28 D, Q Single Supply, Excellent DC Specs

LT1358/LT1359 25 D, Q 600V/µs SR, I

LT1215/LT1216 23 D, Q Single Supply, Excellent DC Specs

LT1211/LT1212 14 D, Q Single Supply, Excellent DC Specs

LT1355/LT1356 12 D, Q 400V/µs SR, I

LT1200/LT1201/LT1202 11 S, D, Q I

LT1217 10 S CFA, I

= 1mA per Amp, Good DC Specs

S

= 1mA, Shutdown

S

Key to Abbreviations:

CFA = Current Feedback Amplifi er
DG = Differential Gain
DP = Differential Phase
MUX = Multiplexer

S = Single
D = Dual
Q = Quad
T = Triple

SR = Slew Rate

= 7.5mA per Amp, Good DC Specs

S

= 5mA per Amp, Good DC Specs

S

= 2.5mA per Amp, Good DC Specs

S

= 1.25mA per Amp, Good DC Specs

S

Note:

Differential gain and phase is measured with a 150Ω load, except for the
LT1203/LT1205 in which case the load is 1000Ω.

VIDEO CABLE DRIVERS

AC-Coupled Video Drivers

When AC-coupling video, the waveform dynamics change
with respect to the bias point of the amplifi er according
to the scene brightness of the video stream. In the worst
case, 1V
or YP

video (composite or Luminance + Sync in Y/C

P-P

format) can exhibit a varying DC content of 0.56V,

BPR

with the dynamic requirement being +0.735V/–0.825V
about the nominal bias level. When this range is ampli­fi ed by two to properly drive a back-terminated cable, the
amplifi er output must be able to swing 3.12V

, thus a

P-P

5V supply is generally required in such circuits, provided
the amplifi er output saturation voltages are suffi ciently
small. The following circuits show various realizations of
AC-coupled video cable drivers.

Figure 1 shows the LT1995 as a single-channel driver. All
the gain-setting resistors are provided on-chip to minimize
part count.

5V

8

47µF

+

47µF

V

IN

M4

9

M2

10

M1

1

P1

2

P2

+

3

P4

LT1995

4

7

75Ω

6

+

5

220µF

10k

V

OUT

f

–3dB

R

L

AN57 F01

= 27MHz

= 75Ω

Figure 1. Single Supply Video Line Driver

Figure 2 shows an LT6551 quad amplifi er driving two sets
of “S-video” (Y/C format) output cables from a single Y/C
source. Internal gain-setting resistors within the LT6551
reduce part-count.

Figure 3 shows the LT6553 ultra-high-speed triple video
driver confi gured for single-supply AC-coupled operation.
This part is ideal for HD or high-resolution workstation ap­plications that demand high bandwidth and fast settling. The
amplifi er gains are factory-set to two by internal resistors.

an57fa

AN57-3

Application Note 57

LT6551

4k

µF

LUMINANCE

CHROMA

470

75Ω

470

75Ω

1k

= 5V

V

CC

4k

µF

1k

1

2

3

4

5

450Ω 450Ω

–

OA

+

450Ω 450Ω

–

OA

+

450Ω 450Ω

–

OA

+

450Ω 450Ω

–

OA

+

AN57 F02

10

9

8

7

6

75Ω

75Ω

75Ω

75Ω

V

CC

CHROMA

OUT1

CHROMA

= 5V

OUT2

LUMINANCE

LUMINANCE

OUT1

S-VIDEO

CONNECTOR

OUT1

S-VIDEO

CONNECTOR

OUT2

OUT2

Figure 2. S-Video Splitter

7V TO 12V

INPUT

22µF*

80.6Ω

2.2k

6.8k

1/3 LT6553

AGND

75Ω

OUTIN

***AVX 12066D226MAT

SANYO 6TPB220ML

220µF**

+

75Ω

AN57 F03

Figure 3. Single Supply Confi guration, One Channel Shown

The LT6557 400MHz triple video driver is specifi cally de­signed to operate in 5V single supply AC-coupled applications
as shown in Figure 4. The input biasing circuitry is contained
on-chip for minimal external component count. A single
resistor programs the biasing level of all three channels.

DC-Coupled Video Drivers

The following circuits show various DC-coupled video
drivers. In DC-coupled systems, the video swings are
fi xed in relation to the supplies used, so back-terminated
cable-drivers need only provide 2V of output range when
optimally biased. In most cases, this permits operation
on lower power supply potential(s) than with AC-coupling
(unclamped mode). Generally DC-coupled circuits use split
supply potentials since the waveforms often include or pass
through zero volts. For single supply operation, the inputs
need to have an appropriate offset applied to preserve linear
amplifi er operation over the intended signal swing.

For systems that lack an available negative supply, the LT1983­3 circuit shown in Figure 5 can be used to easily produce a
local-use –3V that can simplify an overall cable-driving solu­tion, eliminating large output electrolytics, for example.

Figure 6 shows a typical 3-channel video cable driver using
an LT6553. This part includes on-chip gain-setting resis­tors and fl ow-through layout that is optimal for HD and
RGB wideband video applications. This circuit is a good

V

IN

3V TO 5.5V

C

IN

10µF

OFF ON

: TAIYO YUDEN LMK212BJ105

C

FLY

, C

C

IN

: TAIYO YUDEN JMK316BJ106ML

OUT

V

IN

LTC1983-3

SHDN

+

C

C

1µF

V

OUT

GND

C

FLY

Figure 5. –3V at 100mA DC/DC Converter

V

= –3V

OUT

= UP TO 100mA

I

OUT

C

OUT

10µF

–

AN57 F05

BCV

+

5V

V

220µF

OUT R

V+ R

OUT G

V

OUT B

V

75Ω

5V

220µF

75Ω

+

5V

G

220µF

75Ω

+

5V

B

AN57 F04

+
–

500Ω

+
–

500Ω

+
–

500Ω

412Ω

75Ω

75Ω

75Ω

IN R

IN G

IN B

10µF

75Ω

10µF

75Ω

10µF

75Ω

EN

GND

IN R

GND R

IN G

GND G

IN B

GND B

LT6557

500Ω

500Ω

500Ω

Figure 4. 400MHz, AC-Coupled, 5V Single Supply Video Driver

AN57-4

LT6553

–5V

3

4

5

6

7

8

R

IN

75

Ω

G

IN

75Ω

B

IN

75

Ω

+

–

370Ω 370Ω

370Ω 370Ω

–

+

370Ω370Ω

–

+

Figure 6. Triple Video Line Driver

5V

161

152

75Ω

14

75Ω

13

–3V

75Ω

12

11

5V

10

9

–3V

75Ω

75Ω

75Ω

AN57 F06

an57fa

Application Note 57

candidate for the LT1983-3 power solution in systems that
have only 5V available.

Figure 7 shows the LT6551 driving four cables and op­erating from just 3.3V. The inputs need to have signals
centered at 0.83V for best linearity. This application would
be typical of standard-defi nition studio-environment signal
distribution equipment (RGBS format).

Figure 8 shows a simple video splitter application using an
LT6206. Both amplifi ers are driven by the input signal and
each is confi gured for a gain of two, one for driving each

LT6551

R

IN

75

Ω

G

IN

Ω

75

B

IN

Ω

75

SYNC

IN

Ω

75

GND

Figure 7. 3.3V Single Supply LT6551 RGB Plus SYNC
Cable Driver

450Ω 450Ω

–

OA

+

450Ω 450Ω

–

OA

+

450Ω 450Ω

–

OA

+

450Ω 450Ω

–

OA

+

R

OUT

G

OUT

B

OUT

SYNC

3.3V

OUT

75Ω

75Ω

75Ω

75Ω

75Ω

75Ω

75Ω

75Ω

AN57 F07

output cable. Here again careful input biasing is required
(or a negative supply as suggested previously).

Figure 9 shows a means of providing a multidrop tap am­plifi er using the differential input LT6552. This circuit taps
the cable (loop-through confi guration) at a high impedance
and then amplifi es the signal for transmission to a stan­dard 75Ω video load (a display monitor for example). The
looped-through signal would continue on to other locations
before being terminated. The exceptional common mode
rejection of the LT6552 removes any stray noise pickup on
the distribution cable from corrupting the locally displayed
video. This method is also useful for decoupling of ground­loop noise between equipment, such as in automotive
entertainment equipment. To operate on a single supply,
the input signals shown (shield and center of coax feed)
should be non-negative, otherwise a small negative supply
will be needed, such as the local –3V described earlier.

V

IN

5V

3

7

+

V

500Ω

2

–

LT6552

1

REF

DC

8

FB

4

500Ω

R

G

R

8pF

75Ω

6

F

C

F

75Ω

V

AN57 F09

OUT

CABLE

Figure 9. Cable Sense Amplifi er for Loop Through
Connections with DC Adjust

3.3V

499Ω 499Ω

2

V

3

IN

75Ω

5

6

499Ω 499Ω

1µF

8

LT6206

–

+

+

–

4

75Ω

75Ω

1

75Ω

7

75Ω

≈ 50MHz

F

3dB

I

≤ 25mA

S

AN57 F08

Figure 8. Baseband Video Splitter/Cable Driver

V

V

OUT1

OUT2

Clamped AC-Input Video Cable Driver

The circuit in Figure 10 shows a means of driving composite
video on standard 75Ω cable with just a single 3.3V power
supply. This is possible due to the low output saturation
levels of the LT6205 and the use of input clamping to
optimize the bias point of the amplifi er for standard 1V

P-P

source video. The circuit provides an active gain of two
and 75Ω series termination, thus yielding a net gain of
one as seen by the destination load (e.g. display device).
Additional detail on this circuit and other low-voltage
considerations can be found in Design Note 327.

Twisted-Pair Video Cable Driver and Receiver

With the proliferation of twisted-pair wiring practices for
in-building data communication, video transmission on the

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AN57-5

Application Note 57

3.3V

COMPOSITE

VIDEO IN 1V

2.4k

–

+

0.1µF

5

LT6205

2

1k 1k

C1

4.7µF

P–P

BAT54

10k

C2

4.7µF

4

3

470Ω

75Ω

1

≤ 19mA

I

S

VIDEO OUT

75Ω

AN57 F10

Figure 10. Clamped AC-Input Video Cable Driver

same medium offers substantial cost savings compared to
conventional coaxial-cable. Launching a baseband camera
signal into twisted pair is a relatively simple matter of
building a differential driver such as shown in Figure 11.
In this realization one LT6652 is used to create a gain of
+1 and another is used to make a gain of –1. Each output
is series terminated in half the line impedance to provide a
balanced drive condition. An additional virtue of using the
LT6552 in this application is that the incoming unbalanced
signal (from a camera for example) is sensed differentially,
thereby rejecting any ground noise and preventing ground
loops via the coax shield.

At the receiving end of the cable, the signal is terminated
and re-amplifi ed to re-create an unbalanced output for

5V

8

7

FB

1

REF

2

CAMERA VIDEO

INPUT

75Ω

75Ω

1k

–

3

+

8

FB

1

REF

2

–

3

+

Figure 11. Super-Simple Coax to Twisted-Pair Adapter

LT6552

–5V

5V

LT6552

–5V

5

SD

4

7

SD

4

54.9Ω

+

TP

6

TWISTED
PAIR

≈ 110Ω

Z

0

5

54.9Ω

–

TP

6

AN57 F11

connection to display monitors, recorders, etc. The ampli­fi er not only has to provide the 2x gain required for the
output drive, but must also make up for the losses in the
cable run. Twisted pair exhibits a rolloff characteristic that
requires equalization to correct for, so the circuit in Figure
12 shows a suitable feedback network that accomplishes
this. Here again the outstanding common mode rejection
of the LT6552 is harnessed to eliminate stray pickup that
occurs in long cable runs.

EQUALIZATION

–

TP

1V

P–P

BALANCED

+

TP

Figure 12. All-In-One Twisted-Pair Video Line Receiver, Cable
Equalizer, and Display Driver

AN57 F12

100Ω

S1S2

220pF

68pF

150pF

110Ω

10k 10k

768Ω

2.34k

909Ω

10k

1k

1k

5V

8

7

FB

1

REF

2

–

3

+

S1 OPEN, S2 OPEN: NO EQUALIZATION
S1 CLOSED, S2 OPEN: EQUALIZATION FOR ≈ 300ft
S1 OPEN, S2 CLOSED: EQUALIZATION FOR ≈ 700ft
S1 CLOSED, S2 CLOSED: EQUALIZATION FOR ≈ 1000ft

LT6552

–5V

5

SD

6

4

75Ω

VIDEO
OUTPUT
1V

P–P

75Ω

VIDEO PROCESSING CIRCUITS

ADC Driver

Figure 13 shows the LT6554 triple video buffer. This is a
typical circuit used in the digitization of video within high
resolution display units. The input signals (terminations
not shown) are buffered to present low source impedance
and fast settling behavior to ADC inputs that is generally
required to preserve conversion linearity to 10 bits or better.
With high resolution ADCs, it is typical that the settling-time
requirement (if not distortion performance) will call for
buffer bandwidth that far outstrips the baseband signals
themselves in order to preserve the effective number of
[conversion] bits (ENOBs). The 1kΩ loads shown are simply
to represent the ADC input for characterization purposes,
they are not needed in the actual use of the part.

AN57-6

an57fa

Application Note 57

5V

161

R

G

B

–5V

LT6554

3

IN

4

5

IN

6

7

IN

8

+

–

480Ω

480Ω

–

+

480Ω

–

+

152

14

1k

13

–5V

12

11

10

9

1k

5V

1k

–5V

AN57 F13

Figure 13. Triple Video Buffer and A/D Driver

Video Fader

In some cases it is desirable to adjust amplitude of a video
waveform, or cross-fade between two different video
sources. The circuit in Figure 14 provides a simple means
of accomplishing this. The 0V to 2.5V control voltage
provides a steering command to a pair of amplifi er input
sections; at each extreme, one section or the other takes
complete control of the output. For intermediate control
voltages, the inputs each contribute to the output with a
weighting that follows a linear function of control voltage
(e.g. at V

CONTROL

= 1.25V, both inputs contribute at 50%).
The feedback network to each input sets the maximum
gain in the control range (unity gain is depicted in the
example), but depending on the application, other gains
or even equalization functions can be voltage controlled

1

IN1 IN2

2

NULL

3

4

I

C

5

6

7

–

V

0V TO 2.5V

CONTROL

LT1251/LT1256

+

1

–

CONTROL

+

CFS

–

I

I

FS

C

Figure 14. Two-Input Video Fader

14

+

2

13

–

12

2.5VDC

+

INPUT

–

11

I

FS

5k5k

10

9

+

V

8

V

OUT

R

1.5k

R

F2

F1

1.5k

AN57 F14

(see datasheet and Application Note 67 for additional
examples). In the fader example below, it should be noted
that both input streams must be gen-locked for proper
operation, including a black signal (with sync) if fading
to black is intended.

Color Matrix Conversion

Depending on the conventions used by video suppliers in
products targeting specifi c markets, various standards for
color signaling have evolved. Television studios have long
used RGB cameras and monitor equipment to maximize signal
fi delity through the equipment chain. With computer displays
requiring maximum performance to provide clear text and
graphics, the VESA standards also specify an RGB format,
but with separate H and V syncs sent as logic signals. Video
storage and transmission systems, on the other hand, seek
to minimize information content to the extent that perceptual
characteristics of the eye limit any apparent degradation. This
has led to utilizing color-differencing approaches that allowed
reducing bandwidth on the color information channels without
noticeable loss in image sharpness. The consumer 3-chan­nel “component” video connection (YP
sync (Y) plus blue and red axis color-space signals (P

, respectively) that are defi ned as a matrix multiplication

P

R

) has a luma +

BPR

and

B

applied to RGB raw video. The color difference signals are
typically half the spatial resolution of the luma according to
the compression standards defi ned for DVD playback and
digitally broadcast source material, thus lowering “band­width” requirements by some 50%. The following circuits
show methods of performing color-space mappings at the
physical layer (analog domain).

Figure 15 shows a method of generating the standard-defi ­nition YP

signals from an RGB source using a pair of

BPR

LT6550 triple amplifi ers. It should be noted that to ensure
Y includes a correct sync, correct syncs should be present
at all three inputs or else added directly at the Y output
(gated 8.5mA current sink or 350Ω switched to –3.3V).
This circuit does not deliberately reduce bandwidth on the
color component outputs, but most display devices will
nonetheless apply a Nyquist fi lter at the digitizer section
of the “optical engine” in the display unit. The circuit is
shown as DC-coupled, so ideally black level is near ground
for best operation with the low-voltage supplies shown.
Adding input coupling capacitors will allow processing
source video that has substantial offset.

An LT6559 and an LT1395 can also be used to map RGB

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AN57-7

Application Note 57

LT6550

3.3V 3.3V

10

450Ω 450Ω

LT6550

10

450Ω 450Ω

–3.3V

9

8

7

1070Ω

549Ω

2940Ω

R

G

B

1

75Ω

2

75Ω

3

75Ω

–

+

450Ω 450Ω

–

+

450Ω 450Ω

–

+

45

Y = 0.299R + 0.587G + 0.114B

= 0.565(B – Y)

P

B

Figure 15. RGB to YPBPR Component-Video Conversion

signals into YPBPR “component” video as shown in Figure

16. The LT1395 per forms a weighted inverting addition of all
three inputs. The LT1395 output includes an amplifi cation
of the R input by –324/1.07k = –0.3. The amplifi cation of
the G input is by –324/549 = –0.59. Finally, the B input is
amplifi ed by –324/2.94k = – 0.11. Therefore the LT1395
output is –0.3R, –0.59G, –0.11B = –Y. This output is further
scaled and inverted by –301/150 = –2 by LT6559 section
A2, thus producing 2Y. With the division by two that oc­curs due to the termination resistors, the desired Y signal
is generated at the load. The LT6559 section A1 provides
a gain of 2 for the R signal, and performs a subtraction
of 2Y from the section A2 output. The output resistor di­vider provides a scaling factor of 0.71 and forms the 75Ω
back-termination resistance. Thus the signal seen at the
terminated load is the desired 0.71(R – Y) = P

. The LT6559

R

section A3 provides a gain of 2 for the B signal, and also
performs a subtraction of 2Y from the section A2 output.
The output resistor divider provides a scaling factor of 0.57
and forms the 75Ω back-termination resistance. Thus the
signal seen at the terminated load is the desired 0.57(B
– Y) = P

. As with the previous circuit, to develop a normal

B

sync on the Y signal, a normal sync must be inserted on
each of the R, G, and B inputs or injected directly at the Y
output with controlled current pulses.

Figure 17 shows LT6552 amplifi ers connected to convert
component video (YP

) to RGB. This circuit maps the sync

BPR

on Y to all three outputs, so if a separate sync connection is
needed by the destination device (e.g. studio monitor), any
of the R, G, or B channels may be simply looped-through

Ω

1

2

3

P

= 0.713(R – Y)

R

≈ 44MHz

f

3dB

–

+

450Ω 450Ω

–

+

450Ω 450Ω

–

+

45

–3.3V

the sync input (i.e. set Z

105

9

261Ω

75Ω

8

133Ω

7

174Ω

IN

P

R

Y

P

B

AN57 F15

for sync input to unterminated).
This particular confi guration takes advantage of the unique
dual-differential inputs of the LT6552 to accomplish multiple
arithmetic functions in each stage, thereby minimizing the
amplifi er count. This confi guration also processes the wider­bandwidth Y signal through just a single amplifi cation level,
maximizing the available performance. Here again, operation
on low supply voltages is predicated on the absence of
substantial input offset, and input coupling capacitors may
be used if needed (220µF/6V types for example, polarized
according to the input offset condition).

Another realization of a component video (YP

BPR

) to RGB
adapter is shown in Figure 18 using an LT6207. Amplifi er
count is minimized by performing passive arithmetic at the
outputs, but this requires higher gains, thus a higher supply
potential is needed for this (for at least the positive rail).
One small drawback to this otherwise compact solution
is that the Y channel amplifi er must single-handedly drive
all three outputs to produce white, so the helper current
source shown is needed to increase available drive current.
As with the previous circuit, the sync on Y is mapped to
all outputs and input coupling-capacitors can be added if
the input source has signifi cant offset.

Two LT6559s can also be used to map YP

“component”

BPR

video into RGB color space as shown in Figure 19. The Y input
is properly terminated with 75Ω and buffered with a gain of
2 by amplifi er A2. The P
with a gain of 2.8 by amplifi er A1. The P

input is terminated and buffered

R

input is terminated

B

and buffered with a gain of 3.6 by amplifi er A3. Amplifi er B1

an57fa

AN57-8

Application Note 57

75Ω

SOURCES

R

G

B

1070Ω

80.6Ω

549Ω

86.6Ω

2940Ω

76.8Ω

Y = 0.3R + 0.59G + 0.11B

= 0.57(B – Y)

P

B

= 0.71(R – Y)

P

R

ALL RESISTORS 1%

= ±3V TO ±5V

V

S

–

LT1395

+

324Ω

A2

+

A1

1/3 LT6559

–

150Ω 301Ω

–

A2

1/3 LT6559

+

–

A3

1/3 LT6559

+

301Ω

301Ω

301Ω

301Ω

AN57 F16

105Ω

261Ω

75Ω

133

174Ω

P

R

Y

Ω

P

B

Figure 16. High Speed RGB to YPBPR Converter

+3V

499Ω499Ω

8.2pF

8

FB

7

1

REF

2

–

3

+

–3V

Y

P

R

P

B

LT6552

4

21.5Ω

53.6Ω

49.9Ω

25.5Ω

5

SD

6

21.5Ω

11.3Ω

42.2Ω

R = Y + 1.4 • P
G = Y – 0.34 • PB – 0.71 • P
B = Y + 1.8 • P

R

B

+3V

499Ω499Ω

5.6pF

8

FB

7

1

REF

LT6552

2

–

3

8

1

2

3

8

1

2

3

R

5

+

4

SD

–3V

+3V

FB

7

REF

LT6552

–

5

+

4

SD

–3V

+3V

FB

7

REF

LT6552

–

5

+

4

SD

–3V

75Ω

6

909Ω499Ω

2.2pF

75Ω

6

1.3k499Ω

1pF

75Ω

6

BW (± 0.5dB) > 25MHz
BW (–3dB) > 36MHz

≈ 70mA

I

S

G

75Ω

R

75Ω

B

75Ω

AN57 F17

Figure 17. YPBPR to RGB Video Converter

performs an equally weighted addition of amplifi ers A1 and
A2 outputs, thereby producing 2(Y + 1.4P

), which gener-

R

ates the desired R signal at the terminated load due to the
voltage division by 2 caused by the termination resistors.
Amplifi er B3 forms the equally weighted addition of ampli­fi ers A1 and A3 outputs, thereby producing 2(Y + 1.8P

),

B

which generates the desired B signal at the terminated load.
Amplifi er B2 performs a weighted summation of all three
inputs. The P
= 2(–0.34). The P

signal is amplifi ed overall by –301/1.54k • 3.6

B

signal is amplifi ed overall by –301/590

R

• 2.8 = 2(–0.71). The Y signal is amplifi ed overall by 1k/(1k
+ 698) • (1 + [301/(590||1.54k)]) • 2 = 2(1). Therefore

an57fa

AN57-9

Application Note 57

CMPD6001S

4.7k

Y

75Ω

P

B

95.3Ω

P

R

133Ω

174Ω

36Ω

FMMT3906

499Ω165Ω

1

2

3

5

6

499Ω365Ω

7

≈ 40MHz

F

3dB

I

≤ 60mA

S

BLACK LEVELS ≈ 0V

–

+

+

–

5V

LT6207

–5V

1µF

4

13

1µF

16

15

–

14

+

12

+

11

–

10

499Ω

107Ω

80.6Ω

499Ω

R = Y + 1.4 • P
B = Y + 1.8 • P
G = Y – 0.34 • PB – 0.71 • P

150Ω

150Ω

150Ω

150Ω

150Ω

150Ω

R
B

R

R

75Ω

B

75Ω

G

75Ω

AN57 F18

Figure 18. YPBPR to RGB Converter

+

V

–

B1

1/3 LT6559

+

–

V

+

V

–

B2

1/3 LT6559

+

–

V

+

V

–

B3

1/3 LT6559

+

–

V

301Ω

301Ω

324Ω

Ω

75

R

55

Ω

75

G

R = Y + 1.4 • P
G = Y – 0.34 • PB – 0.71 • PR
B = Y + 1.77 • P

V+/V– = ±3V

75

Ω

B

5

R

B

AN57 F19

165Ω

P

R

75Ω

301Ω

Y

75Ω

118Ω

P

B

75Ω

301Ω

+

V

–

A1

1/3 LT6559

+

–

V

301Ω

+

V

–

A2

1/3 LT6559

+

55

–

V

301Ω

+

V

–

A3

1/3 LT6559

+

5

–

V

301Ω

590Ω

1.54k

698

324Ω

1k

1k

Ω

1k

1k

1k

Figure 19. High Speed YPBPR to RGB Converter

the amplifi er B2 output is 2(Y – 0.34PB – 0.71PR), which
generates the desired G signal at the terminated load. Like
the previous circuits shown, sync present on the Y input is
reconstructed on all three R, G, and B outputs.

Video Inversion

The circuit in Figure 20 is useful for viewing photographic
negatives on video. A single channel can be used for com­posite or monochrome video. The inverting amplifi er stages

AN57-10

are only switched in during active video so the blanking, sync
and color burst (if present) are not disturbed. To prevent
video from swinging negative, a voltage offset equal to the
peak video signal is added to the inverted signal.

Graphics Overlay Adder

Multiplexers that provide pixel-speed switching are also
useful in providing simple graphics overlay, such as time­stamps or logo “bugs”. Figure 21 shows an LT1675 pair

an57fa