LT1395/LT1396/LT1397
O U
W
U
PPLICATI
S I FOR ATIO
A
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.
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.
The G input also arrives via 75Ω coax and adds its
contributiontotheYsignalviaa432ΩresistorR9, which
is tied to the inverting input of amplifier A2. There is also
a 90.9Ω 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.
IfwenowsumthethreecontributionsattheoutputofA2,
we get:
A2OUT = 3.40Y – 1.02R – 0.37B
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:
A2OUT = (1.02R – 1.02R) + 2G + (0.37B – 0.37B)
= 2G
Buffered Color-Difference to RGB Matrix
Theback-terminationresistorR11thenhalvestheoutput
of A2 resulting in the G output.
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 75Ω
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.
R1
1k
Y
R2
R5
+
1k
75Ω
A1
R-Y
R
1/2 LT1396
–
R3
267Ω
R4
88.7Ω
R6
205Ω
R11
75Ω
+
The B output is also a back-terminated 75Ω signal
created using resistor R16 and amplifier A3 configured
foragainof+4viaresistorsR14andR15.Thenoninverting
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.
R7
1k
A2
LT1395
G
–
R10
267Ω
R8
261Ω
R9
698Ω
B-Y
R12
1k
R16
75Ω
+
A3
R13
1k
B
1/2 LT1396
–
R14
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
267Ω
ALL RESISTORS 1%
= ± 5V
V
S
R15
88.7Ω
1395/6/7 F05
Figure 5. Buffered Color-Difference to RGB Matrix
139567fc
12
Linear [ Linear ]
