L7UHEN2
9
LT1190
Output Shutdown
Output Shutdown
Cable Terminations
The LT1190 operational amplifier has been optimized as a
low cost video cable driver. The ±50mA guaranteed output
current enables the LT1190 to easily deliver 7.5V
P-P
into
100Ω, while operating on ±5V supplies or 2.6V
P-P
on a
single 5V supply.
APPLICATIO S I FOR ATIO
WUUU
FREQUENCY (Hz)
CLOSED-LOOP VOLTAGE GAIN (dB)
10
8
6
4
2
0
–2
–4
–6
–8
–10
LT1190 • TA08
100M10M1M100k
V
S
= ±5V
T
A
= 25°C
A
V
= 2
R
FB
= 1k
R
G
= 330Ω
A
V
= 1
R
FB
= 1k
R
G
= 1k
CABLE
2
+
–
5V
7
LT1190
–5V
4
36
Cable Driver Voltage Gain vs Frequency
Double Terminated Cable Driver
R
G
75Ω
75Ω
R
FB
When driving a cable it is important to terminate the cable
to avoid unwanted reflections. This can be done in one of
two ways: single termination or double termination. With
single termination, the cable must be terminated at the
receiving end (75Ω to ground) to absorb unwanted en-
ergy. The best performance can be obtained by double
termination (75Ω in series with the output of the amplifier,
and 75Ω to ground at the other end of the cable). This
termination is preferred because reflected energy is ab-
sorbed at each end of the cable. When using the double
termination technique it is important to note that the signal
is attenuated by a factor of 2, or 6dB. This can be compen-
sated for by taking a gain of 2, or 6dB in the amplifier. The
cable driver has a – 3dB bandwidth in excess of 30MHz
while driving the 150Ω load.
Using the Shutdown Feature
The LT1190 has a unique feature that allows the amplifier
to be shut down for conserving power or for multiplexing
several amplifiers onto a common cable. The amplifier will
shut down by taking Pin 5 to V
–
. In shutdown, the
amplifier dissipates 15mW while maintaining a true high
impedance output state of 15kΩ in parallel with the
feedback resistors. The amplifiers must be used in a
noninverting configuration for MUX applications. In in-
verting configurations the input signal is fed to the output
through the feedback components. The following scope
photos show that with very high R
L
, the output is truly high
impedance; the output slowly decays toward ground.
Additionally, when the output is loaded with as little as 1kΩ
the amplifier shuts off in 400ns. This shutoff can be under
the control of HC CMOS operating between 0V and – 5V.
LT1190 • TA09
1MHz SINE WAVE GATED OFF WITH
SHUTDOWN PIN, A
V
= 1, R
L
= SCOPE PROBE
V
OUT
–5V
0V
V
SHDN
LT1190 • TA10
1MHz SINE WAVE GATED OFF WITH
SHUTDOWN PIN, A
V
= 1, R
L
= 1kΩ
V
OUT
–5V
0V
V
SHDN