Datasheet

LT1395/LT1396/LT1397
11
139567fd
operation, 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 amplifi er will remain enabled at all times, then
the EN pin should be tied to the V
–
supply. The enable pin
current is approximately 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.
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 approxi-
mately 40ns (50% point to 50% point) (Figure 3).
APPLICATIONS INFORMATION
Figure 4. Buffered RGB to Color-Difference Matrix
–
+
A2
1/4 LT1397
–
+
A3
1/4 LT1397
–
+
A1
1/4 LT1397
R7
255Ω
R6
127Ω
R5
255Ω
R10
2320Ω
R9
432Ω
R11
82.5Ω
R
G
B
R12
90.9Ω
R13
76.8Ω
ALL RESISTORS 1%
V
S
= ±5V
R8
845Ω
75Ω
SOURCES
R1
255Ω
R2
255Ω
R4
255Ω
R3
255Ω
B-Y
Y
R-Y
1395/6/7 F04
–
+
A4
1/4 LT1397
Differential Input Signal Swing
To avoid any breakdown condition on the input transis-
tors, the differential input swing must be limited to ±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
An LT1397 can be used to create buffered color-difference
signals from RGB inputs (Figure 4). In this application,
the R input arrives via 75Ω coax. It is routed to the non-
inverting input of LT1397 amplifi er 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 amplifi er (A2),
which also sums the weighted G and B inputs to create a
–0.5 • Y output. LT1397 amplifi er A3 then takes the
–0.5 • Y output and amplifi es it by a gain of –2, resulting
in the Y output. Amplifi er A1 is confi gured in a noninvert-
ing gain of 2 with the bottom of the gain resistor R2 tied
to the Y output. The output of amplifi er A1 thus results
in the color-difference output R-Y.
The B input is similar to the R input. It arrives via 75Ω
coax, and is routed to the noninverting input of LT1397
amplifi er A4, and to a 2320Ω resistor R10. There is also
a 76.8Ω termination resistor R13, which yields a 75Ω
V
S
= ±5V
V
IN
= 1V
R
F
= 255Ω
R
G
= 255Ω
R
L
= 100Ω
1395/6/7 F02
OUTPUT
EN
V
S
= ±5V
V
IN
= 1V
R
F
= 255Ω
R
G
= 255Ω
R
L
= 100Ω
1395/6/7 F03
OUTPUT
EN
Figure 2. Amplifi er Enable Time, A
V
= 2
Figure 3. Amplifi er Disable Time, A
V
= 2