Datasheet
LT1991
20
1991fh
1991 F13
V
S
–
V
S
+
V
S
–
V
S
+
V
S
–
V
S
+
V
S
–
V
S
+
M9
M3
M1
P1
P3
P9
OUT
V
CC
V
OUT
V
EE
REF
LT1991
8
9
10
1
2
3
7
6
5
4
M9
M3
M1
P1
P3
P9
OUT
V
CC
V
OUT
V
EE
REF
LT1991
8
9
10
1
2
3
7
6
5
4
M9
M3
M1
P1
P3
P9
OUT
V
CC
V
OUT
V
EE
REF
LT1991
8
9
10
1
2
3
7
6
5
4
M9
M3
M1
P1
P3
P9
OUT
V
CC
V
OUT
V
EE
REF
LT1991
8
9
10
1
2
3
7
6
5
4
GAIN = 2 GAIN = 5
GAIN = 7 GAIN = 8
GAIN = 11
V
IN
+
V
IN
–
V
IN
+
V
IN
–
V
IN
+
V
IN
–
V
IN
+
V
IN
–
V
S
–
V
S
+
M9
M3
M1
P1
P3
P9
OUT
V
CC
V
OUT
V
EE
REF
LT1991
8
9
10
1
2
3
7
6
5
4
V
IN
+
V
IN
–
–
+
R
F
R
G
R
G
V
IN
+
V
IN
+
V
IN
–
V
IN
–
V
OUT
V
OUT
V
OUT
= GAIN • (V
IN
+
– V
IN
–
)
GAIN = R
F
/R
G
–
+
1991 F12
50k
150k
450k
50k
150k
450k
450k
450k
8
6
5
9
10
1
2
3
LT1991
CLASSICAL DIFFERENCE AMPLIFIER
CLASSICAL DIFFERENCE AMPLIFIER IMPLEMENTED
WITH LT1991. R
F
= 450k, R
G
= 150k, GAIN = 3.
GAIN CAN BE ADJUSTED BY "CROSS COUPLING." MAKING THE
DASHED CONNECTIONS REDUCE THE GAIN FROM 3 T0 2.
WHEN CROSS COUPLING, SEE WHAT IS CONNECTED TO THE
V
IN
+
VOLTAGE. CONNECTING P3 AND M1 GIVES +3 –1 = 2.
CONNECTIONS TO V
IN
–
ARE SYMMETRIC: M3 AND P1.
R
F
CROSS-
COUPLING
M9
M3
M1
P1
P3
P9
4pF
4pF
Difference Amplifier: Additional Integer Gains Using
Cross-Coupling
Figure 12 shows the basic difference amplifier as well as
the LT1991 in a difference gain of 3. But notice the effect
of the additional dashed connections. This is referred to
as “cross-coupling” and has the effect of reducing the
differential gain from 3 to 2. Using this method, additional
integer gains are achievable, as shown in Table 5 below,
so that all integer
gains from 1 to 13 are achieved with the
LT1991. Note that the equations can be written by inspection
from the V
IN
+
connections, and that the V
IN
–
connections
are simply the opposite (swap P for M and M for P). Noise
gain, bandwidth, and input impedance specifications for the
various cases are also tabulated, as these are not obvious.
Schematics are provided in Figure 13.
Figure 13. Integer Gain Difference
Amplifiers Using Cross-Coupling
Figure 12. Another Method of Selecting Difference Gain Is “Cross-Coupling.”
The Additional Method Means the LT1991 Provides All Integer Gains from 1 to 13
Table 5. Connections Using Cross-Coupling. Note That Equations Can
Be Written by Inspection of the V
IN
+
Column
Gain
V
IN
+
V
IN
–
Equation
Noise
Gain
–3dB BW
kHz
R
IN
+
Typ kΩ
R
IN
–
Typ kΩ
2 P3, M1 M3, P1 3 – 1 5 70 281 141
5 P9, M3, M1 M9, P3, P1 9 – 3 – 1 14 32 97 49
6* P9, M3 M9, P3 9 – 3 13 35 122 49
7 P9, P1, M3 M9, M1, P3 9 + 1 – 3 14 32
121 44
8 P9, M1 M9, P1 9 – 1 11 38 248 50
11 P9, P3, M1 M9, M3, P1 9 + 3 – 1 14 32 242 37
*Gain of 6 is better implemented as shown previously, but is included here for completeness.
APPLICATIONS INFORMATION