LT1991 Linear Technology, LT1991 Datasheet - Page 12

• Manufacturer Part Number: LT1991
• Description: Precision / 100uA Gain Selectable Amplifier
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT1991
with V
max V
op amp output will go more negative from V
amount V
Or:
The most negative that V
The situation where this function is negative, and there-
fore problematic, when V
been dealt with in Figure 1. The strength of the equation
is demonstrated in that it provides the three solutions
suggested in Figure 1: raise V
some negative V
Likewise, from the lower common mode extreme, mak-
ing the negative input more negative will raise the output
voltage, limited by V
Again, the additional input range calculated here is only
available provided the other remaining constraint is not
violated, the maximum voltage allowed on the pin.
The Classical Noninverting Amplifi er: High Input Z
Perhaps the most common op amp confi guration is the
noninverting amplifi er. Figure 4 shows the textbook
12
V
V
Max V
MIN V
OUT
MORE
EXT
MORE
MAX OR MIN
= V
MORE
MORE
MORE
(the high cm limit), as V
= (V
V
REF
MORE
= 0, the op amp output is at V
Figure 3. Calculating Additional
Voltage Range of Inverting Inputs
V
EXT
REF
= (V
= (V
• R
– V
MORE
– V
F
MORE
REF
REF
/R
R
R
CC
G
G
OUT
G
.
V
– V
– V
, so:
INT
– 0.04V.
• R
) • R
OUT
REF
EE
CC
–
+
F
/R
– 0.04V) • R
= 0 and V
can go is V
+ 0.04V) • R
G
REF
/R
G
V
V
R
CC
EE
F
F
MORE
, lower V
R
(should be negative)
(should be positive)
F
EE
goes positive, the
EE
V
= 0, has already
G
G
REF
/R
EE
/R
REF
+ 0.04V, so:
F
, or provide
F
. From the
REF
1991 F03
by the
representation of the circuit on the top. The LT1991 is
shown on the bottom confi gured in a precision gain of 5.5.
One of the benefi ts of the noninverting op amp confi gura-
tion is that the input impedance is extremely high. The
LT1991 maintains this benefi t. Given the fi nite number of
available feedback resistors in the LT1991, the number of
gain confi gurations is also fi nite. The complete list of such
Hi-Z input noninverting gain confi gurations is shown in
Table 1. Many of these are also represented in Figure 5 in
schematic form. Note that the P-side resistor inputs have
been connected so as to match the source impedance
seen by the internal op amp inputs. Note also that gain
and noise gain are identical, for optimal precision.
Figure 4. The LT1991 as a Classical Noninverting Op Amp
V
IN
CLASSICAL NONINVERTING OP AMP CONFIGURATION.
YOU PROVIDE THE RESISTORS.
CLASSICAL NONINVERTING OP AMP CONFIGURATION
IMPLEMENTED WITH LT1991. R
GAIN IS ACHIEVED BY GROUNDING, FLOATING OR FEEDING BACK
THE AVAILABLE RESISTORS TO ARRIVE AT DESIRED R
WE PROVIDE YOU WITH <0.1% RESISTORS.
10
8
9
1
2
3
R
150k
450k
450k
150k
50k
50k
G
V
IN
–
+
450k
R
–
+
F
4pF
F
V
GAIN = 1 + R
450k
= 225k, R
4pF
OUT
www.DataSheet4U.com
5
= GAIN • V
G
LT1991
= 50k, GAIN = 5.5.
F
/R
V
OUT
IN
G
6
F
AND R
1991 F04
V
G
OUT
.
1991fg