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參數(shù)資料
| 型號(hào)： | LBMM |
| 廠商： | Linear Integrated Systems |
| 英文描述： | Precision, 100UA Gain Selectable Amplifier |
| 中文描述： | 精密，100uA的可選增益放大器 |
| 文件頁數(shù)： | 10/24頁 |
| 文件大小： | 311K |
| 代理商： | LBMM |
LT1991
10
1991fb
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 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.
Figure 3. Calculating Additional
Voltage Range of Inverting Inputs
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 therefore
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 sug-
gested in Figure 1: raise V
REF
, lower V
EE
, or provide some
negative V
MORE
.
Likewise, from the lower common mode extreme, making
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)
Figure 4. The LT1991 as a Classical Noninverting Op Amp
APPLICATIOU
W
U
U
–
+
V
REF
R
F
R
F
R
G
R
G
1991 F03
V
MAX OR MIN
V
INT
V
MORE
V
CC
V
EE
–
+
R
F
R
G
V
IN
V
IN
V
OUT
V
OUT
V
= GAIN V
IN
GAIN = 1 + R
F
/R
G
–
+
1991 F04
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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
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
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