LT1187IN8 データシートの表示（PDF） - Linear Technology
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LT1187IN8 Datasheet PDF : 16 Pages
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LT1187
APPLICATIONS INFORMATION
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
feedback. 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:
(RFB + RG)/RG.
Like the single-ended case, the differential voltage gain is
set by the external resistors: (RFB + RG)/RG. The maximum
input differential signal for which the output will respond
is approximately ±0.38V.
S/D
V+
VIN
3
2
1
5
+7
– LT1187
+/REF
8 –/FB 4
V–
RFB
6
VOUT
RG
AV
=
+
RFB + RG
RG
S/D
V+
3+ 5 7
VIN
2
1
– LT1187
+/REF
8 –/FB 4
V–
RFB
6
VOUT
RG
AV
=
–
RFB +RG
RG
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 amplifier 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 VOS at Pin 8. The complete
expression for the output offset voltage is:
VOUT = (VOS + IOS(RS) + IB(RREF)) • (RFB + RG)/RG + IB(RFB)
RS represents the input source resistance, typically 75Ω,
and RREF represents the finite source impedance from the
DC reference voltage, for VREF grounded, RREF = 0Ω. The
IOS is normally a small contributor and the expression
simplifies to:
VOUT = VOS(RFB + RG)/RG + IB(RFB)
If RFB is limited to 1k the last term of the equation con-
tributes only 2mV, since IB is less than 2µA.
7 V+
6
VIN DIFF
VIN
S/D
V+
3+ 5 7
2 – LT1187
1 +/REF
8 –/FB 4
V–
RFB
S/D
V+
6
VOUT
VIN
VIN DIFF
RG
3
2
5
+7
– LT1187
1 +/REF
8 –/FB 4
V–
RFB
6
VOUT
RG
VO = (VIN DIFF + VIN) RFB + RG
RG
( ( VO =
RFB +RG
RG
VIN DIFF –
RFB
RG
VIN
LT1187 • AI01
Q1 Q2
3+ 2–
RE
1.1k
RS
RS
345mA
Q3 Q4
+ 1 REF
RE
1.1k
RREF
350mA
RFB
8
RG
4 V–
ILT1187 • F01
Figure 1. Simplified Input Stage Schematic
1187fa
8
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