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(yè)數(shù)： 19/24頁(yè)
文件大?。?/td> 311K
代理商： LBMM
LT1991
19
1991fb
High Voltage CM Difference Amplifiers
This class of difference amplifier remains to be discussed.
Figure 14 shows the basic circuit on the top. The effective
input voltage range of the circuit is extended by the fact
that resistors R
T
attenuate the common mode voltage
seen by the op amp inputs. For the LT1991, the most
useful resistors for R
G
are the M1 and P1 450k
resistors,
because they do not have diode clamps to the supplies and
therefore can be taken outside the supplies. As before, the
input CM of the op amp is the limiting factor and is set by
the voltage at the op amp +input, V
INT
. By superposition
we can write:
V
INT
= V
EXT
(R
F
||R
T
)/(R
G
+ R
F
||R
T
) + V
REF
(R
G
||R
T
)/
(R
F
+ R
G
||R
T
) + V
TERM
(R
F
||R
G
)/(R
T
+ R
F
||R
G
)
Solving for V
EXT
:
V
EXT
= (1 + R
G
/(R
F
||R
T
)) (V
INT
– V
REF
(R
G
||R
T
)/
(R
F
+ R
G
||R
T
) – V
TERM
(R
F
||R
G
)/(R
T
+ R
F
||R
G
))
Given the values of the resistors in the LT1991, this
equation has been simplified and evaluated, and the re-
sulting equations provided in Table 6. As before, substi-
tuting V
CC
– 1.2 and V
EE
+ 1 for V
LIM
will give the valid
upper and lower common mode extremes respectively.
Following are sample calculations for the case shown in
Figure 14, right-hand side. Note that P9 and M9 are
terminated so row 3 of Table 6 provides the equation:
MAX V
EXT
= 11 (V
CC
– 1.2V) – V
REF
– 9 V
TERM
= 11 (10.8V) – 2.5 – 9 12 = 8.3V
and:
MIN V
EXT
= 11 (V
EE
+ 1V) – V
REF
– 9 V
TERM
= 11 (1V) – 2.5 – 9 12 = –99.5V
but this exceeds the 60V absolute maximum rating of the
P1, M1 pins, so –60V becomes the de facto negative
common mode limit. Several more examples of high CM
circuits are shown in Figures 15, 16, 17 for various
supplies.
Figure 14. Extending CM Input Range
Table 6. HighV CM Connections Giving Difference Gains
for the LT1991
Max, Min V
EXT
(Substitute V
CC
– 1.2,
V
EE
+ 1 for V
LIM
)
2 V
LIM
- V
REF
5 V
LIM
– V
REF
– 3 V
TERM
11 V
LIM
– V
REF
– 9 V
TERM
14 V
LIM
– V
REF
– 12 V
TERM
Noise
Gain
2
5
11
14
Gain
1
1
1
1
V
IN+
P1
P1
P1
P1
V
IN–
M1
M1
M1
M1
R
T
P3, M3
P9, M9
P3||P9
M3||M9
APPLICATIOU
W
U
U
–
+
R
F
R
G
R
G
V
IN
+
(= V
EXT
)
V
IN
+
V
IN
–
V
IN
–
V
OUT
V
OUT
V
= GAIN (V
IN
GAIN = R
F
/R
G
+
– V
IN
–
)
–
+
1991 F14
50k
150k
450k
50k
150k
450k
450k
450k
8
7
4
6
5
9
10
1
2
3
LT1991
REF
HIGH CM VOLTAGE DIFFERENCE AMPLIFIER
INPUT CM TO OP AMP IS ATTENUATED BY
RESISTORS R
T
CONNECTED TO V
TERM.
HIGH NEGATIVE CM VOLTAGE DIFFERENCE AMPLIFIER
IMPLEMENTED WITH LT1991.
R
F
= 450k, R
G
= 450k, R
T
50k, GAIN = 1
V
TERM
= V
CC
= 12V, V
REF
= 2.5V, V
EE
= GROUND.
R
F
M9
M3
M1
P1
P3
P9
V
CC
V
EE
R
T
R
T
V
TERM
V
REF
INPUT CM RANGE
= –60V TO 8.3V
12V
2.5V
4pF
4pF
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