LT1395/LT1396/LT1397
O U
S
W
U
PPLICATI
A
I FOR ATIO
Differential Input Signal Swing
OUTPUT
To avoid any breakdown condition on the input transis-
tors, thedifferentialinputswingmustbelimitedto±5V. In
normal operation, the differential voltage between the
input pins is small, so the ±5V limit is not an issue.
Buffered RGB to Color-Difference Matrix
EN
An LT1397 can be used to create buffered color-differ-
ence signals from RGB inputs (Figure 4). In this applica-
tion, the R input arrives via 75Ω coax. It is routed to the
noninverting input of LT1397 amplifier A1 and to a 845Ω
resistor R8. There is also an 82.5Ω termination resistor
R11, which yields a 75Ω input impedance at the R input
when considered in parallel with R8. R8 connects to the
inverting input of a second LT1397 amplifier (A2), which
also sums the weighted G and B inputs to create a
–0.5 • Y output. LT1397 amplifier A3 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.
1395/6/7 F02
VS = ±5V
VIN = 1V
R
F = 255Ω
RL = 100Ω
RG = 255Ω
Figure 2. Amplifier Enable Time, AV = 2
OUTPUT
EN
1395/6/7 F03
VS = ± 5V RF = 255Ω
IN = 1V G = 255Ω
RL = 100Ω
The B input is similar to the R input. It arrives via 75Ω
coax, and is routed to the noninverting input of LT1397
amplifier A4, and to a 2320Ω resistor R10. There is also
a 76.8Ω termination resistor R13, which yields a 75Ω
V
R
Figure 3. Amplifier Disable Time, AV = 2
it is important to keep the EN pin at least 3V below the V+
supply. If a V+ of less than 3V is desired, and the amplifier
will remain enabled at all times, then the EN pin should be
tied to the V– supply. The enable pin current is approxi-
mately 30µA when activated. If using CMOS open-drain
logic, an external 1k pull-up resistor is recommended to
ensure that the LT1395CS6 remains disabled in spite of
any CMOS drain leakage currents.
+
75Ω
R8
A1
SOURCES
R-Y
845Ω
1/4 LT1397
R
–
R1
255Ω
R11
82.5Ω
R9
432Ω
R7
G
B
255Ω
R12
90.9Ω
R10
2320Ω
–
R6
127Ω
R5
R2
255Ω
255Ω
R13
76.8Ω
A2
1/4 LT1397
+
–
A3
Y
The enable/disable times are very fast when driven from
standard 5V CMOS logic. The LT1395CS6 enables in
about 30ns (50% point to 50% point) while operating on
±5V supplies (Figure 2). Likewise, the disable time is
approximately40ns(50%pointto50%point)(Figure3).
1/4 LT1397
+
R4
255Ω
R3
255Ω
–
ALL RESISTORS 1%
= ±5V
A4
B-Y
V
S
1/4 LT1397
1395/6/7 F04
+
Figure 4. Buffered RGB to Color-Difference Matrix
139567fc
11
Linear [ Linear ]
