Datasheet

LT1880
9
1880fa
APPLICATIONS INFORMATION
–
+
V
REF
V
IN
R
G
R
F
INVERTING: A
V
= –R
F
/R
G
OP AMP INPUTS DO NOT MOVE,
BUT ARE FIXED AT DC BIAS
POINT V
REF
INPUT DOES NOT HAVE TO BE
RAIL-TO-RAIL
–
+
V
REF
V
IN
R
G
R
F
NONINVERTING: A
V
= 1 +R
F
/R
G
INPUTS MOVE BY AS MUCH AS
V
IN
, BUT THE OUTPUT MOVES
MORE
INPUT MAY NOT HAVE TO BE
RAIL-TO-RAIL
–
+
V
IN
NONINVERTING: A
V
= +1
INPUTS MOVE AS MUCH AS
OUTPUT
INPUT MUST BE
RAIL-TO-RAIL FOR OVERALL
CIRCUIT RAIL-TO-RAIL
PERFORMANCE
1880 F01
Figure 1. Some Op Amp Confi gurations Do Not Require
Rail-to Rail Inputs to Achieve Rail-to-Rail Outputs
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 40μV input offset
voltage contribution, plus the additional error due to input
bias current times the ~100k effective source impedance,
contribute only negligibly to error.
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
B
= 150pA typical) and is therefore appli-
cable to this demanding task. Figure 3 shows an LT1880
configured as a transimpedance photodiode amplifier.
1.5V
–1.5V
100k, 0.1%
1M, 0.1%
V
IN
±1.35V
OUTPUT
SWING
±13.5V SWINGS
WELL OUTSIDE
SUPPLY RAILS
–
+
LT1880
1880 F02
–
+
OUT
C
F
R
F
51.1k
5V
–5V
C
D
PHOTODIODE
(SEE TEXT)
LT1880
WORST-CASE
OUTPUT OFFSET
≤196μV AT 25°C
≤262μV 0°C TO 70°C
≤323μV –40°C TO 85°C
1880 F02
Figure 2. Extreme Inverting Case: Circuit Operates Properly with
Input Voltage Swing Well Outside Op Amp Supply Rails.
Figure 3. Precision Photodiode Amplifi er