LT1991 Linear Technology, LT1991 Datasheet - Page 21

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

APPLICATIONS INFORMATION
High Voltage CM Difference Amplifi ers
This class of difference amplifi er 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
by the op amp inputs. For the LT1991, the most useful
resistors for R
cause 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
we can write:
Solving for V
Given the values of the resistors in the LT1991, this equa-
tion has been simplifi ed and evaluated, and the resulting
equations provided in Table 6. As before, substituting V
– 1.2 and V
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:
and:
but this exceeds the 60V absolute maximum rating of
the P1, M1 pins, so –60V becomes the de facto nega-
tive common mode limit. Several more examples of high
CM circuits are shown in Figures 15, 16, 17 for various
supplies.
V
(R
V
(R
MAX V
MIN V
INT
EXT
F
F
+ R
+ R
= V
= (1 + R
EXT
EXT
G
G
EXT
||R
||R
EE
= 11 • (V
EXT
= 11 • (V
T
T
G
T
• (R
+ 1 for V
) + V
) – V
G
attenuate the common mode voltage seen
:
are the M1 and P1 450kΩ resistors, be-
/(R
F
||R
TERM
TERM
F
= 11 • (10.8V) – 2.5 – 9 • 12 = 8.3V
||R
= 11 • (1V) – 2.5 – 9 • 12 = –99.5V
EE
T
CC
)/(R
LIM
T
+ 1V) – V
)) • (V
– 1.2V) – V
• (R
• (R
G
will give the valid upper and
+ R
F
F
||R
||R
INT
F
G
G
||R
REF
)/(R
)/(R
– V
INT
T
REF
) + V
– 9 • V
T
REF
T
. By superposition
+ R
+ R
– 9 • V
• (R
REF
F
F
||R
||R
TERM
G
• (R
G
||R
G
TERM
)
))
T
G
)/
||R
T
CC
)/
Table 6. HighV CM Connections Giving Difference Gains for
the LT1991
Gain
INPUT CM RANGE
1
1
1
1
= –60V TO 8.3V
V
IN
–
V
(= V
P1
P1
P1
P1
IN
V
+
V
V
EXT
IN
IN
IN
+
–
+
)
Figure 14. Extending CM Input Range
V
12V
M1
M1
M1
M1
IN
V
–
R
R
TERM
G
G
10
HIGH NEGATIVE CM VOLTAGE DIFFERENCE AMPLIFIER
IMPLEMENTED WITH LT1991.
R
V
8
9
1
2
3
M3||M9
R
F
TERM
P3, M3
P9, M9
P3||P9
HIGH CM VOLTAGE DIFFERENCE AMPLIFIER
INPUT CM TO OP AMP IS ATTENUATED BY
RESISTORS R
T
= 450k, R
M9
M3
M1
P1
P3
P9
R
T
= V
150k
450k
450k
150k
50k
50k
CC
R
T
G
= 12V, V
= 450k, R
Noise
Gain
T
11
14
2
5
CONNECTED TO V
–
+
REF
T
= 2.5V, V
www.DataSheet4U.com
V
50k, GAIN = 1
V
14 • V
R
R
11 • V
CC
EE
–
+
5 • V
F
F
7
4
4pF
(Substitute V
V
GAIN = R
450k
450k
OUT
4pF
LIM
LIM
V
EE
LIM
Max, Min V
2 • V
EE
TERM.
= GROUND.
= GAIN • (V
– V
– V
+ 1 for V
– V
F
LIM
LT1991
/R
REF
REF
REF
LT1991
G
- V
V
V
CC
– 3 • V
– 12 • V
– 9 • V
OUT
REF
REF
IN
REF
EXT
LIM
– 1.2,
+
21
– V
5
)
6
TERM
IN
TERM
TERM
1991 F14
–
V
2.5V
1991fg
)
OUT