Datasheet
LT1991
21
1991fh
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 450kΩ resistors, be-
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
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 equa-
tion has been simplified and evaluated, and the resulting
equations provided in Table 6. As before, substituting
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.
–
+
R
F
R
G
R
G
V
IN
+
(= V
EXT
)
V
IN
+
V
IN
–
V
IN
–
V
OUT
V
OUT
V
OUT
= GAIN • (V
IN
+
– V
IN
–
)
GAIN = R
F
/R
G
–
+
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
Table 6. HighV CM Connections Giving Difference Gains for
the LT1991
Gain
V
IN
+
V
IN
–
R
T
Noise
Gain
Max, Min V
EXT
(Substitute V
CC
– 1.2,
V
EE
+ 1 for V
LIM
)
1 P1 M1 2 2 • V
LIM
- V
REF
1 P1 M1 P3, M3 5 5 • V
LIM
– V
REF
– 3 • V
TERM
1 P1 M1 P9, M9 11 11 • V
LIM
– V
REF
– 9 • V
TERM
1 P1 M1 P3||P9
M3||M9
14 14 • V
LIM
– V
REF
– 12 • V
TERM
Figure 14. Extending CM Input Range
APPLICATIONS INFORMATION