LT1880 LINER [Linear Technology], LT1880 Datasheet - Page 9

• Manufacturer Part Number: LT1880
• Description: SOT-23, Rail-to-Rail Output, Picoamp Input Current Precision Op Amp
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The circuit of Figure 2 shows an extreme example of the
inverting case. The input voltage at the 1M resistor can
swing 13.5V and the LT1880 will output an inverted,
divided-by-ten version of the input voltage. The input
accuracy is limited by the resistors to 0.2%. Output
referred, this error becomes 2.7mV. The 40 V input offset
voltage contribution, plus the additional error due to input
bias current times the ~100k effective source impedance,
contribute only negligibly to error.
Figure 2. Extreme Inverting Case: Circuit Operates Properly
with Input Voltage Swing Well Outside Op Amp Supply Rails.
WELL OUTSIDE
13.5V SWINGS
SUPPLY RAILS
V
IN
V
V
REF
IN
1M, 0.1%
U
INVERTING: A
OP AMP INPUTS DO NOT MOVE,
BUT ARE FIXED AT DC BIAS
POINT V
INPUT DOES NOT HAVE TO BE
RAIL-TO-RAIL
R
G
REF
U
100k, 0.1%
–
V
+
= –R
–
LT1880
+
–1.5V
F
1.5V
/R
G
Figure 1. Some Op Amp Configurations Do Not Require
Rail-to Rail Inputs to Achieve Rail-to-Rail Outputs
R
W
F
OUTPUT
SWING
1.35V
U
V
IN
NONINVERTING: A
INPUTS MOVE BY AS MUCH AS
V
MORE
INPUT MAY NOT HAVE TO BE
RAIL-TO-RAIL
IN
V
REF
, BUT THE OUTPUT MOVES
R
G
–
+
Precision Photodiode Amplifier
Photodiode amplifiers usually employ JFET op amps be-
cause of their low bias current; however, when precision
is required, JFET op amps are generally inadequate due to
their relatively high input offset voltage and drift. The
LT1880 provides a high degree of precision with very low
bias current (I
cable to this demanding task. Figure 3 shows an LT1880
configured as a transimpedance photodiode amplifier.
R
V
F
= 1 + R
F
/R
WORST-CASE
OUTPUT OFFSET
G
PHOTODIODE
196 V AT 25 C
262 V 0 C TO 70 C
323 V –40 C TO 85 C
(SEE TEXT)
Figure 3. Precision Photodiode Amplifier
B
V
IN
= 150pA typical) and is therefore appli-
NONINVERTING: A
INPUTS MOVE AS MUCH AS
OUTPUT
INPUT MUST BE RAIL-TO-
RAIL FOR OVERALL CIRCUIT
RAIL-TO-RAIL PERFORMANCE
C
D
–
+
–
+
R
V
LT1880
F
= +1
C
51.1k
–5V
F
5V
LT1880
OUT
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