LT1996
19
1996f
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 LT1996, the most
useful resistors for R
G
are the M9 and P9 50kΩ 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 LT1996, 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 P81 and M81 are
terminated so row 3 of Table 6 provides the equation:
MAX V
EXT
= 91/9 • (V
CC
– 1.2V) – V
REF
/9 – 9 • V
TERM
= (10.11) • (10.8) – 0.11(2.5) – 9(10) =
18.9V
and:
MIN V
EXT
= 91/9 • (V
EE
+ 1V) – V
REF
/9 – 9 • V
TERM
= (10.11)(1) – 0.11(2.5) – 9(10) = –80.2V
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.
Figure 14. Extending CM Input Range
4pF
4pF
–
+
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
–
+
1996 F14
450k
450k
8
7
4
6
5
9
10
1
2
3
LT1996
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 LT1996.
R
F
= 450k, R
G
= 50k, R
T
5.55k, GAIN = 9
V
TERM
= 10V = V
CC
= 12V, V
REF
= 2.5V, V
EE
= 0V.
R
F
V
CC
V
EE
R
T
R
T
V
TERM
V
REF
INPUT CM RANGE
= –60V TO 18.9V
10V
12V
2.5V
450k/81
450k/27
450k/9
450k/81
450k/27
450k/9
4pF
4pF
Table 6. HighV CM Connections Giving Difference Gains
for the LT1996
Max, Min V
EXT
Noise (Substitute V
CC
– 1.2,
Gain V
IN
+
V
IN
–
R
T
Gain V
EE
+ 1 for V
LIM
)
9 P9 M9 10 10/9 • V
LIM
- V
REF
/9
9 P9 M9 P27, M27 37 37/9 • V
LIM
– V
REF
/9 – 3 • V
TERM
9 P9 M9 P81, M81 91 91/9 • V
LIM
– V
REF
/9 – 9 • V
TERM
9 P9 M9 P27||P81 118 118/9 • V
LIM
– V
REF
/9 – 12 • V
TERM
M27||M81
APPLICATIO S I FOR ATIO
WUU
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