LT1996 Linear Technology, LT1996 Datasheet - Page 19

• Manufacturer Part Number: LT1996
• Description: 100uA Gain Selectable Amplifier
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
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
seen by the op amp inputs. For the LT1996, the most
useful resistors for R
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
we can write:
Solving for V
Given the values of the resistors in the LT1996, this
equation has been simplified and evaluated, and the re-
sulting equations provided in Table 6. As before, substi-
tuting V
upper and lower common mode extremes respectively.
Following are sample calculations for the case shown in
Figure 14, right-hand side. Note that P81 and M81 are
terminated so row 3 of Table 6 provides the equation:
and:
but this exceeds the 60V absolute maximum rating of the
P9, M9 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.
4
V
(R
V
(R
MAX V
MIN V
INT
EXT
F
F
+ R
+ R
= V
= (1 + R
CC
EXT
EXT
G
G
EXT
– 1.2 and V
||R
||R
EXT
= 91/9 • (V
= (10.11) • (10.8) – 0.11(2.5) – 9(10) =
18.9V
= 91/9 • (V
= (10.11)(1) – 0.11(2.5) – 9(10) = –80.2V
T
T
• (R
) – V
) + V
G
T
:
/(R
attenuate the common mode voltage
F
U
||R
TERM
TERM
F
G
||R
T
are the M9 and P9 50kΩ resistors,
EE
)/(R
T
CC
EE
)) • (V
• (R
• (R
U
+ 1 for V
+ 1V) – V
G
– 1.2V) – V
+ R
F
F
||R
||R
INT
F
G
G
||R
)/(R
)/(R
– V
LIM
W
INT
T
REF
) + V
REF
. By superposition
T
T
REF
will give the valid
/9 – 9 • V
+ R
+ R
/9 – 9 • V
• (R
REF
F
F
||R
||R
G
• (R
U
G
||R
G
)
))
TERM
G
T
)/
||R
TERM
T
)/
Table 6. HighV CM Connections Giving Difference Gains
for the LT1996
Gain
INPUT CM RANGE
9
9
9
9
= –60V TO 18.9V
V
IN
–
(= V
V
P9
P9
P9
P9
IN
V
+
V
V
EXT
IN
IN
IN
+
–
+
)
Figure 14. Extending CM Input Range
10V
V
M9
M9
M9
M9
IN
V
R
R
TERM
–
G
G
10
HIGH NEGATIVE CM VOLTAGE DIFFERENCE AMPLIFIER
IMPLEMENTED WITH LT1996.
R
V
8
9
1
2
3
R
F
TERM
HIGH CM VOLTAGE DIFFERENCE AMPLIFIER
INPUT CM TO OP AMP IS ATTENUATED BY
RESISTORS R
M27||M81
T
= 450k, R
P27, M27
P81, M81
P27||P81
R
450k/81
450k/27
= 10V = V
450k/27
450k/81
450k/9
450k/9
T
R
T
G
= 50k, R
Noise
T
Gain
CC
118 118/9 • V
10
37
91
CONNECTED TO V
= 12V, V
–
+
T
5.55k, GAIN = 9
37/9 • V
91/9 • V
V
V
12V
R
R
CC
EE
–
+
F
F
REF
7
4
(Substitute V
V
GAIN = R
450k
= 2.5V, V
450k
10/9 • V
OUT
4pF
4pF
4pF
4pF
V
LIM
LIM
LIM
Max, Min V
EE
TERM.
= GAIN • (V
– V
– V
– V
+ 1 for V
F
EE
LIM
/R
REF
REF
REF
LT1996
G
LT1996
= 0V.
- V
/9 – 3 • V
/9 – 9 • V
/9 – 12 • V
V
CC
V
REF
REF
OUT
EXT
LIM
IN
REF
– 1.2,
+
)
– V
/9
19
5
6
IN
TERM
TERM
1996 F14
–
V
2.5V
TERM
)
OUT
1996f