LT1187 Linear Technology, LT1187 Datasheet - Page 9

• Manufacturer Part Number: LT1187
• Description: Low Power Video Difference Amplifier
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Operating with Low Closed-Loop Gains
The LT1187 has been optimized for closed-loop gains of
2 or greater. For a closed-loop gain of 2 the response
peaks about 2dB. Peaking can be eliminated by placing a
capacitor across the feedback resistor, (feedback zero).
This peaking shows up as time domain overshoot of
about 25%.
A
V
= 2, OVERSHOOT = 25%, R
Closed-Loop Voltage Gain vs Frequency
–1
9
8
7
6
5
4
3
2
1
0
100k
Small-Signal Transient Response
V
T
A
R
R
A
S
V
FB
G
= 25°C
= 2
= 900
= ±5V
= 900
U U
C
1M
FB
FREQUENCY (Hz)
= 5pF
C
FB
C
FB
FB
= R
10M
= 0pF
= 10pF
G
= 1k
W
LT1187 • AI03
100M
LT1187 • AI04
U
Extending the Input Range
Figure 1 shows a simplified schematic of the LT1187. In
normal operation the REF pin 1 is grounded or taken to a
DC offset control voltage and differential signals are ap-
plied between pins 2 and 3. The input responds linearly
until all of the 345 A current flows through the 1.1k
resistor and Q1 (or Q2) turns off. Therefore the maximum
input swing is 380mV
tial pair, Q3 and Q4, is running at slightly larger current so
that when the first input stage limits, the second stage
remains biased to maintain the feedback.
Occasionally it is necessary to handle signals larger than
760mV
tricked to handle up to 1.5V
to ground pin 3 and apply the differential input signal
between pin 1 and 2. The input signal is now applied
across two 1.1k resistors in series. Since the input signal
is applied to both input pairs, the first pair will run out of
bias current before the second pair, causing the amplifier
to go open-loop. The results of this technique are shown
in the following scope photo.
P-P
A
V
= 2, WITH 8pF FEEDBACK CAPACITOR
at the input. The LT1187 input stage can be
Small-Signal Transient Response
P
or 760mV
P-P
. To do this, it is necessary
P-P
. The second differen-
LT1187 • AI05
LT1187
9