12

LT1395/LT1396/LT1397

139567fc

S

APPLICATI

I FOR ATIO

input impedance when considered in parallel with R10.

R10 also connects to the inverting input of amplifier A2,

adding the B contribution to the Y signal as discussed

above. Amplifier A4 is configured in a noninverting gain

of 2 configuration with the bottom of the gain resistor R4

tied to the Y output. The output of amplifier A4 thus

results in the color-difference output B-Y.

The G input also arrives via 75

Ω coax and adds its

contribution to the Y signal via a 432

Ω resistor R9, which

is tied to the inverting input of amplifier A2. There is also

a 90.9

Ω termination resistor R12, which yields a 75Ω

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.

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 5). The R output is a back-terminated 75

Ω

signal created using resistor R5 and amplifier A1 config-

ured for a gain of +4 via 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 +4 via 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 respectively, 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

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 5. 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

88.7

Ω

ALL RESISTORS 1%

–

+

A1

1/2 LT1396

R3

267

Ω

R

R5

75

Ω

R4

88.7

Ω

1395/6/7 F05