Datasheet

LT1991
12
1991fh
with V
MORE
= 0, the op amp output is at V
REF
. From the
max V
EXT
(the high cm limit), as V
MORE
goes positive, the
op amp output will go more negative from V
REF
by the
amount V
MORE
• R
F
/R
G
, so:
V
OUT
= V
REF
– V
MORE
• R
F
/R
G
Or:
V
MORE
= (V
REF
– V
OUT
) • R
G
/R
F
The most negative that V
OUT
can go is V
EE
+ 0.04V, so:
Max V
MORE
= (V
REF
– V
EE
– 0.04V) • R
G
/R
F
(should be positive)
The situation where this function is negative, and there-
fore problematic, when V
REF
= 0 and V
EE
= 0, has already
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
REF
, lower V
EE
, or provide
some negative V
MORE
.
Likewise, from the lower common mode extreme, mak-
ing the negative input more negative
will raise the output
voltage, limited by V
CC
– 0.04V.
MIN V
MORE
= (V
REF
– V
CC
+ 0.04V) • R
G
/R
F
(should be negative)
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 Amplifier: High Input Z
Perhaps the most common op amp configuration is the
noninverting amplifier. Figure 4 shows the textbook
representation of the circuit on the top. The LT1991 is
shown on the bottom configured in a
precision gain
of 5.5. One of the benefits of the noninverting op amp
configuration is that the input impedance is extremely
high. The LT1991 maintains this benefit. Given the finite
number of available feedback resistors in the LT1991, the
number of gain configurations is also finite. The complete
list of such Hi-Z input noninverting gain configurations 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.
–
+
V
REF
R
F
R
F
R
G
R
G
1991 F03
V
EXT
MAX OR MIN
V
INT
V
MORE
V
CC
V
EE
–
+
R
F
R
G
V
IN
V
IN
V
OUT
V
OUT
V
OUT
= GAIN • V
IN
GAIN = 1 + R
F
/R
G
–
+
1991 F04
50k
150k
450k
50k
150k
450k
450k
450k
8
6
5
9
10
1
2
3
LT1991
CLASSICAL NONINVERTING OP AMP CONFIGURATION.
YOU PROVIDE THE RESISTORS.
CLASSICAL NONINVERTING OP AMP CONFIGURATION
IMPLEMENTED WITH LT1991. R
F
= 225k, R
G
= 50k, GAIN = 5.5.
GAIN IS ACHIEVED BY GROUNDING, FLOATING OR FEEDING BACK
THE AVAILABLE RESISTORS TO ARRIVE AT DESIRED R
F
AND R
G
.
WE PROVIDE YOU WITH <0.1% RESISTORS.
4pF
4pF
Figure 3. Calculating Additional
Voltage Range of Inverting Inputs
Figure
4. The LT1991 as a Classical Noninverting Op Amp
APPLICATIONS INFORMATION