LT1991
W U U
U
APPLICATIO S I FOR ATIO
VEXT (the high cm limit), as VMORE goes positive, the op representation of the circuit on the top. The LT1991 is
ampoutputwillgomorenegativefromVREF bytheamount shown on the bottom configured in a precision gain of 5.5.
VMORE • RF/RG, so:
One of the benefits of the noninverting op amp configura-
tion 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
OUT = VREF – VMORE • RF/RG
Or:
VMORE = (VREF – VOUT) • RG/RF
The most negative that VOUT can go is VEE + 0.04V, so:
Max VMORE = (VREF – VEE – 0.04V) • RG/RF
(should be positive)
Thesituationwherethisfunctionisnegative,andtherefore
problematic, when VREF = 0 and VEE = 0, has already been
dealt with in Figure 1. The strength of the equation is
demonstrated in that it provides the three solutions sug-
gested in Figure 1: raise VREF, lower VEE, or provide some
R
F
R
G
–
+
negative VMORE
.
V
OUT
Likewise, from the lower common mode extreme, making
the negative input more negative will raise the output
voltage, limited by VCC – 0.04V.
V
IN
V
= GAIN • V
GAIN = 1 + R /R
OUT
IN
G
F
CLASSICAL NONINVERTING OP AMP CONFIGURATION.
YOU PROVIDE THE RESISTORS.
MIN VMORE = (VREF – VCC + 0.04V) • RG/RF
(should be negative)
R
F
50k
450k
4pF
8
V
CC
R
G
G
150k
450k
9
V
–
+
MORE
10
V
INT
V
EXT
MAX OR MIN
–
+
6
R
V
OUT
V
450k
150k
50k
EE
1
2
3
V
REF
R
F
1991 F03
4pF
Figure 3. Calculating Additional
Voltage Range of Inverting Inputs
450k
LT1991
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.
5
V
IN
CLASSICAL NONINVERTING OP AMP CONFIGURATION
IMPLEMENTED WITH LT1991. R = 225k, R = 50k, GAIN = 5.5.
F
G
GAIN IS ACHIEVED BY GROUNDING, FLOATING OR FEEDING BACK
THE AVAILABLE RESISTORS TO ARRIVE AT DESIRED R AND R .
F
G
The Classical Noninverting Amplifier: High Input Z
1991 F04
WE PROVIDE YOU WITH <0.1% RESISTORS.
Perhaps the most common op amp configuration is the
noninverting amplifier. Figure 4 shows the textbook
Figure 4. The LT1991 as a Classical Noninverting Op Amp
1991fb
10
Linear Systems [ Linear Systems ]
