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LT1187CS8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT1187
7
1187fa
TYPICAL PERFORMANCE CHARACTERISTICS
Harmonic Distortion vs
Output Voltage
Large-Signal Transient Response
Output Voltage Step vs
Settling Time, A
V
= 2
LOAD RESISTANCE (Ω)
10
–5
OUTPUT VOLTAGE SWING (V)
–3
1
5
100 1000
LT1187 • TPC19
3
–1
T
A
= –55°C
T
A
= 125°C
V
S
= ±5V
T
A
= 125°C
T
A
= –55°C
T
A
= 25°C
T
A
= 25°C
TEMPERATURE (°C)
SLEW RATE (V/ s)
LT1187 • TPC20
µ
–50
150
200
250
–25 0 50 7525
100 125
–SLEW RATE
+SLEW RATE
V
S
= ±5V
R
L
= 1k
V
O
= ±0.5V
A
V
= 2
SETTLING TIME (ns)
40
–4
OUTPUT VOLTAGE STEP (V)
–2
2
4
50 100
LT1187 • TPC21
60 70 80 90
V
S
= ±5V
T
A
= 25°C
R
L
= 1k
0
10mV
10mV
OUTPUT VOLTAGE (V
P-P
)
0
DISTORTION (dBc)
–30
–35
–40
–45
–50
–55
–60
35
LT1187 • TPC22
12
467
HD
3
HD
2
V
S
= ±5V
T
A
= 25°C
R
L
= 1k
f = 1MHz
A
V
= 10
INPUT IN LIMITING, A
V
= 3, SR = 180V/µs
LT1187 • TPC23
Small-Signal Transient Response Small-Signal Transient Response
A
V
= 2, R
FB
= 1k, OVERSHOOT = 25%
LT1187 • TPC24
A
V
= 2, R
FB
= 1k, OVERSHOOT = 25%
LT1187 • TPC25
Output Voltage Swing vs
Load Resistance
Slew Rate vs Temperature
LT1187
8
1187fa
APPLICATIONS INFORMATION
The primary use of the LT1187 is in converting high speed
differential signals to a single-ended output. The LT1187
video difference amplifi er has two uncommitted high
input impedance (+) and (–) inputs. The amplifi er has
another set of inputs which can be used for reference and
feedback. Additionally, this set of inputs give gain adjust
and DC control to the difference amplifi er. The voltage
gain of the LT1187 is set like a conventional operational
amplifi er. Feedback is applied to Pin 8, and it is optimized
for gains of 2 or greater. The amplifi er can be operated
single-ended by connecting either the (+) or (–) inputs to
the +/REF (Pin 1). The voltage gain is set by the resistors:
(R
FB
+ R
G
)/R
G
.
Like the single-ended case, the differential voltage gain is
set by the external resistors: (R
FB
+ R
G
)/R
G
. The maximum
input differential signal for which the output will respond
is approximately ±0.38V.
Power Supply Bypassing
The LT1187 is quite tolerant of power supply bypassing.
In some applications a 0.1µF ceramic disc capacitor placed
1/2 inch from the amplifi er is all that is required. In ap-
plications requiring good settling time, it is important to
use multiple bypass capacitors. A 0.1µF ceramic disc in
parallel with a 4.7µF tantalum is recommended.
Calculating the Output Offset Voltage
Both input stages contribute to the output offset voltage at
Pin 6. The feedback correction forces balance in the input
stages by introducing an input V
OS
at Pin 8. The complete
expression for the output offset voltage is:
V
OUT
= (V
OS
+ I
OS
(R
S
) + I
B
(R
REF
)) • (R
FB
+ R
G
)/R
G
+ I
B
(R
FB
)
R
S
represents the input source resistance, typically 75Ω,
and R
REF
represents the fi nite source impedance from the
DC reference voltage, for V
REF
grounded, R
REF
= 0Ω. The
I
OS
is normally a small contributor and the expression
simplifi es to:
V
OUT
= V
OS
(R
FB
+ R
G
)/R
G
+ I
B
(R
FB
)
If R
FB
is limited to 1k the last term of the equation con-
tributes only 2mV, since I
B
is less than 2µA.
V
+
7
6
LT1187
V
–
4
1
8
OUT
V
+
3
–
2
IN
V
S/D
5
A
V
= +
R
FB
+
R
FB
7
6
LT1187
4
1
8
OUT
V
+
3
–
2
IN DIFF
V
S/D
5
R
FB
LT1187 • AI01
7
6
LT1187
4
+/REF
1
–/FB
8
OUT
V
+
3
–
2
S/D
5
V
O
=
R
FB
+
R
FB
7
6
LT1187
4
1
8
OUT
V
+
3
–
2
IN
V
S/D
5
A
V
= –
R
FB
+
R
FB
IN
V
IN
V
(
(
V
IN DIFF
R
FB
(
(
V
IN
+/REF
–/FB
+/REF
–/FB
+/REF
–/FB
R
G
R
G
R
G
V
O
= (V
R
FB
+
IN DIFF
+ V
IN
)
R
G
R
G
R
G
IN DIFF
V
R
G
R
G
R
G
R
G
–
R
G
R
G
R
G
V
+
V
–
V
+
V
–
V
+
V
–
ILT1187 • F01
+
1
REF
R
REF
R
G
8
R
FB
6
350 A
+
3
R
S
345 A
–
2
R
S
7V
+
R
E
1.1k
R
E
1.1k
4V
–
Q1 Q2 Q3 Q4
Figure 1. Simplifi ed Input Stage Schematic
LT1187
9
1187fa
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%.
APPLICATIONS INFORMATION
Closed-Loop Voltage Gain vs Frequency
FREQUENCY (Hz)
CLOSED-LOOP VOLTAGE GAIN (dB)
9
LT1187 • AI03
100M10M1M100k
7
5
–1
1
3
V
S
= ±5V
T
A
= 25°C
A
V
= 2
R
FB
= 900Ω
R
G
= 900Ω
C
FB
= 0pF
0
2
4
6
8
C
FB
= 10pF
C
FB
= 5pF
Small-Signal Transient Response
A
V
= 2, OVERSHOOT = 25%, R
FB
= R
G
= 1k
LT1187 • AI04
Small-Signal Transient Response
A
V
= 2, WITH 8pF FEEDBACK CAPACITOR
LT1187 • AI05
Extending the Input Range
Figure 1 shows a simplifi ed schematic of the LT1187. In
normal operation REF, Pin 1, is grounded or taken to a DC
offset control voltage and differential signals are applied
between Pins 2 and 3. The input responds linearly until
all of the 345µA current fl ows through the 1.1k resistor
and Q1 (or Q2) turns off. Therefore the maximum input
swing is 380mV
P
or 760mV
P-P
. The second differential
pair, Q3 and Q4, is running at slightly larger current so
that when the fi rst input stage limits, the second stage
remains biased to maintain the feedback.
Occasionally it is necessary to handle signals larger than
760mV
P-P
at the input. The LT1187 input stage can be
tricked to handle up to 1.5V
P-P
. To do this, it is necessary
to ground Pin 3 and apply the differential input signal
between Pins 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 fi rst pair will run out of
bias current before the second pair, causing the amplifi er
to go open loop. The results of this technique are shown
in the following scope photo.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT1187CS8#PBF

Mfr. #:
Buy LT1187CS8#PBF
Manufacturer:
Analog Devices Inc.
Description:
Video Amplifiers L Pwr Video Diff Amp
Lifecycle:
New from this manufacturer.
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