LT1189 Просмотр технического описания (PDF) - Linear Technology
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LT1189 Datasheet PDF : 12 Pages
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LT1189
APPLICATI S I FOR ATIO
The primary use of the LT1189 is in converting high speed
differential signals to a single-ended output. The LT1189
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 differential amplifier. The voltage gain of
the LT1189 is set like a conventional operational amplifier.
Feedback is applied to pin 8, and it is optimized for gains
of 10 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 ±170mV.
S/D
V+
VIN
35
2+
7
1
– LT1189
+/REF
8 –/FB 4
V–
RFB
6
VOUT
RG
AV = + RFB + RG
RG
S/D
V+
VIN
35
2+
7
1
– LT1189
+/REF
8 –/FB 4
V–
RFB
6
VOUT
RG
AV
=
–
RFB +RG
RG
Power Supply Bypassing
The LT1189 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
applications 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 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
contributes only 2mV since IB is less than 2µA.
7 V+
VIN DIFF
VIN
S/D
V+
35
2+
7
– LT1189
1 +/REF
8 –/FB 4
V–
RFB
S/D
V+
6
VOUT
VIN
VIN DIFF
RG
35
2+
7
1
– LT1189
+/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
LT1189 • AI01
6
Q1 Q2
3+ 2–
RE
300
RS
RS
345µA
Q3 Q4
+ 1 REF
RE
300
RREF
350µA
RFB
8
RG
4 V–
LT1189 • AI02
Figure 1. Simplified Input Stage Schematic
8
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