9

LT1395/LT1396/LT1397

S

APPLICATI

I FOR ATIO

termination when considered in parallel with R9. Using

superposition, it is straightforward to determine the

output of amplifier A2. Although inverted, it sums the R,

G and B signals in the standard proportions of 0.3R,

0.59G and 0.11B that are used to create the Y signal.

Amplifier A3 then inverts and amplifies the signal by 2,

resulting in the Y output.

is attenuated via resistors R6 and R7 such that amplifier

A2’s noninverting input sees 0.83Y. Using superposition,

we can calculate the positive gain of A2 by assuming that

R8 and R9 are grounded. This results in a gain of 2.41 and

a contribution at the output of A2 of 2Y. The R-Y input is

amplified by A2 with the gain set by resistors R8 and R10,

giving an amplification of –1.02. This results in a contri-

bution at the output of A2 of 1.02Y – 1.02R. The B-Y input

is amplified by A2 with the gain set by resistors R9 and

R10, giving an amplification of – 0.37. This results in a

contribution at the output of A2 of 0.37Y – 0.37B.

If we now sum the three contributions at the output of A2,

we get:

It is important to remember though that Y is a weighted

sum of R, G and B such that:

Y = 0.3R + 0.59G + 0.11B

If we substitute for Y at the output of A2 we then get:

= 2G

The back-termination resistor R11 then halves the output

of A2 resulting in the G output.

Figure 2. Buffered Color-Difference to RGB Matrix

–

+

A2

LT1395

R7

1k

B-Y

R-Y

Y

R10

267

Ω

R11

75

Ω

R6

205

Ω

R2

1k

R1

1k

R8

261

Ω

R9

698

Ω

–

+

A3

1/2 LT1396

R14

267

Ω

B

G

R16

75

Ω

R12

1k

R13

1k

R15

267

Ω

ALL RESISTORS 1%

–

+

A1

1/2 LT1396

R3

267

Ω

R

R5

75

Ω

R4

267

Ω

1395/6/7 F02

Figure 1. Buffered RGB to Color-Difference Matrix

A2

1/4 LT1397

A3

1/4 LT1397

–

+

A1

1/4 LT1397

R7

255

Ω

R6

127

Ω

R5

255

Ω

R10

2320

Ω

R9

432

Ω

R11

82.5

Ω

R

G

B

R12

90.9

Ω

R13

76.8

Ω

ALL RESISTORS 1%

R8

845

Ω

75

Ω

SOURCES

R1

255

Ω

R2

255

Ω

R4

255

Ω

R3

255

Ω

B-Y

Y

R-Y

1395/6/7 F01

A4

1/4 LT1397

Buffered Color-Difference to RGB Matrix

An LT1395 combined with an LT1396 can be used to

create buffered RGB outputs from color-difference sig-

nals (Figure 2). The R output is a back-terminated 75

Ω

signal created using resistor R5 and amplifier A1 config-

ured for a gain of +2 via 255

Ω resistors R3 and R4. The

noninverting input of amplifier A1 is connected via 1k

resistors R1 and R2 to the Y and R-Y inputs respectively,

resulting in cancellation of the Y signal at the amplifier

input. The remaining R signal is then amplified by A1.

The B output is also a back-terminated 75

Ω signal

created using resistor R16 and amplifier A3 configured

for a gain of +2 via 255

Ω resistors R14 and R15. The

noninverting input of amplifier A3 is connected via 1k

resistors R12 and R13 to the Y and B-Y inputs respec-

tively, resulting in cancellation of the Y signal at the

amplifier input. The remaining B signal is then amplified

by A3.

The G output is the most complicated of the three. It is a

weighted sum of the Y, R-Y and B-Y inputs. The Y input