12

LT1398/LT1399/LT1399HV

S

APPLICATI

I FOR ATIO

Buffered RGB to Color-Difference Matrix

Two LT1398s can be used to create buffered color-

difference signals from RGB inputs (Figure 8). In this

application, the R input arrives via 75

Ω coax. It is routed

to the noninverting input of LT1398 amplifier A1 and to

a 1082

Ω resistor R8. There is also an 80.6Ω termination

A2

1/2 LT1398

B1

1/2 LT1398

–

+

A1

1/2 LT1398

R7

324

Ω

R6

162

Ω

R5

324

Ω

R10

2940

Ω

R9

549

Ω

R11

80.6

Ω

R

G

B

R12

86.6

Ω

R13

76.8

Ω

ALL RESISTORS 1%

R8

1082

Ω

75

Ω

SOURCES

R1

324

Ω

R2

324

Ω

R4

324

Ω

R3

324

Ω

B-Y

Y

R-Y

1398/99 F08

B2

1/2 LT1398

resistor R11, which yields a 75

Ω input impedance at the

R input when considered in parallel with R8. R8 connects

to the inverting input of a second LT1398 amplifier (A2),

which also sums the weighted G and B inputs to create a

–0.5 • Y output. LT1398 amplifier B1 then takes the

–0.5 • Y output and amplifies it by a gain of –2, resulting

in the Y output. Amplifier A1 is configured in a noninvert-

ing gain of 2 with the bottom of the gain resistor R2 tied

to the Y output. The output of amplifier A1 thus results in

the color-difference output R-Y.

The B input is similar to the R input. It arrives via 75

Ω

coax, and is routed to the noninverting input of LT1398

amplifier B2, and to a 2940

Ω resistor R10. There is also

a 76.8

Ω termination resistor R13, which yields a 75Ω

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 B2 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 B2 thus

results in the color-difference output B-Y.

Figure 7. LT1399HV Output Voltage Swing

50ns/DIV

1398/99 F07

Figure 8. Buffered RGB to Color-Difference Matrix