LT1991 Datasheet, PDF (19/24 Pages) Linear Integrated Systems – Precision, 100UA Gain Selectable Amplifier

LT1991
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_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,
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.
Table 6. HighV CM Connections Giving Difference Gains
for the LT1991
Gain V_IN+
1 P1
1 P1
1 P1
1 P1
Max, Min V_EXT
Noise (Substitute V_CC – 1.2,
V_IN–
R_T
Gain
V_EE + 1 for V_LIM)
M1
2
2 • V_LIM - V_REF
M1 P3, M3 5 5 • V_LIM – V_REF – 3 • V_TERM
M1 P9, M9 11 11 • V_LIM – V_REF – 9 • V_TERM
M1 P3||P9 14 14 • V_LIM – V_REF – 12 • V_TERM
M3||M9
R_F
V_IN^–
V_IN⁺
(= V_EXT)
V_CC
R_G
–
R_G
R_T
V_OUT
+
V_OUT = GAIN • (V_IN⁺ – V_IN^–)
R_T
V_EE ^{GAIN = RF/RG}
R_F
V_REF
V_TERM
HIGH CM VOLTAGE DIFFERENCE AMPLIFIER
INPUT CM TO OP AMP IS ATTENUATED BY
RESISTORS R_T CONNECTED TO V_TERM.
12V
8 M9 ^50k
9 ^{M3 150k}
10 M1 ^450k
V_IN^– V_IN⁺
INPUT CM RANGE
= –60V TO 8.3V
1 P1 ^450k
2 P3 ^150k
3 P9 ^50k
7
450k
4pF
–
+
4pF
6
V_OUT
450k
4
REF 5 _2.5V
LT1991
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.
1991 F14
Figure 14. Extending CM Input Range
1991fb
19