LT1187 Linear Technology, LT1187 Datasheet - Page 8

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

APPLICATIO S I FOR ATIO
LT1187
The primary use of the LT1187 is in converting high speed
differential signals to a single-ended output. The LT1187
video difference amplifier has two uncommitted high input
impedance (+) and (–) inputs. The amplifier has another
set of inputs which can be used for reference and feed-
back. Additionally, this set of inputs give gain adjust and
DC control to the difference amplifier. The voltage gain of
the LT1187 is set like a conventional operational amplifier.
Feedback is applied to pin 8, and it is optimized for gains
of 2 or greater. The amplifier 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
Like the single-ended case, the differential voltage gain is
set by the external resistors: (R
input differential signal for which the output will respond
is approximately 0.38V.
8
V
IN DIFF
V
IN
V
IN
3
2
1
8
3
2
1
8
R
R
G
G
–/FB
+
–
+/REF
–/FB
+
–
+/REF
S/D
S/D
LT1187
LT1187
V
5
5
A
O
V
V
V
R
V
V
R
= (V
FB
+
–
= +
+
–
FB
7
4
7
4
IN DIFF
R
FB
R
+
G
6
6
R
U U
+ V
G
IN
V
V
)
OUT
OUT
R
FB
R
V
+
G
IN
R
G
FB
V
R
IN DIFF
G
+ R
V
V
O
IN
=
W
G
(
R
)/R
3
2
1
8
R
FB
3
2
1
8
G
R
+
+
–
–/FB
G
+/REF
+
–
–/FB
S/D
+/REF
R
G
S/D
LT1187
G
. The maximum
LT1187
5
A
5
(
V
V
V
R
V
R
V
V
= –
IN DIFF
+
–
FB
FB
7
4
FB
+
–
7
4
R
+ R
FB
U
R
–
+
G
( (
6
R
R
6
R
G
LT1187 • AI01
G
FB
G
)/R
V
V
V
OUT
IN
OUT
G
.
Power Supply Bypassing
The LT1187 is quite tolerant of power supply bypassing.
In some applications a 0.1 F ceramic disc capacitor
placed 1/2 inch from the amplifier is all that is required. In
applications requiring good settling time, it is important to
use multiple bypass capacitors. A 0.1 F ceramic disc in
parallel with a 4.7 F 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
expression for the output offset voltage is:
V
R
and R
DC reference voltage, for V
I
simplifies to:
V
If R
contributes only 2mV, since I
OS
OUT
OUT
S
3
R
represents the input source resistance, typically 75 ,
+
S
is normally a small contributor and the expression
FB
= (V
= V
REF
2
is limited to 1k the last term of the equation
R
OS
–
OS
S
represents the finite source impedance from the
Figure 1. Simplified Input Stage Schematic
(R
Q1
+ I
345 A
FB
1.1k
OS
R
E
+R
(R
Q2
G
S
)/R
) + I
+
G
B
+ I
(R
1 REF
R
REF
REF
B
REF
(R
B
grounded, R
)) (R
Q3
FB
OS
is less than 2 A.
350 A
)
1.1k
R
E
at pin 8. The complete
Q4
FB
+R
G
7 V
4 V
REF
8
)/R
+
–
G
= 0 . The
+ I
R
ILT1187 • AI02
G
B
R
(R
FB
6
FB
)