LT1991 Linear Technology, LT1991 Datasheet - Page 20

• Manufacturer Part Number: LT1991
• Description: Precision / 100uA Gain Selectable Amplifier
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT1991
Difference Amplifi er: Additional Integer Gains Using
Cross-Coupling
Figure 12 shows the basic difference amplifi er 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
are simply the opposite (swap P for M and M for P). Noise
gain, bandwidth, and input impedance specifi cations for the
various cases are also tabulated, as these are not obvious.
Schematics are provided in Figure 13.
Table 5. Connections Using Cross-Coupling. Note That Equations Can
Be Written by Inspection of the V
Gain
*Gain of 6 is better implemented as shown previously, but is included here for completeness.
20
6*
11 P9, P3, M1 M9, M3, P1 9 + 3 – 1
2
5
7
8
P9, M3, M1 M9, P3, P1 9 – 3 – 1
P9, P1, M3 M9, M1, P3 9 + 1 – 3
P3, M1
P9, M3
P9, M1
V
IN
+
IN
+
connections, and that the V
V
V
M3, P1
M9, P3
M9, P1
IN
IN
V
–
+
IN
–
R
R
CLASSICAL DIFFERENCE AMPLIFIER
G
G
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.
Equation
3 – 1
9 – 3
9 – 1
IN
+
–
+
Column
Noise
R
R
Gain
F
F
14
13
14
11
14
5
V
GAIN = R
OUT
–3dB BW
= GAIN • (V
kHz
70
32
35
32
38
32
F
/R
IN
G
V
–
OUT
IN
Typ kΩ
connections
+
R
281
122
121
248
242
– V
97
IN
IN
+
–
)
Typ kΩ
R
141
49
49
44
50
37
IN
–
COUPLING
V
V
IN
CROSS-
IN
–
+
V
V
V
V
IN
IN
IN
IN
–
+
–
+
CLASSICAL DIFFERENCE AMPLIFIER IMPLEMENTED
WITH LT1991. R
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
CONNECTIONS TO V
IN
+
10
VOLTAGE. CONNECTING P3 AND M1 GIVES +3 –1 = 2.
8
9
1
2
3
P1
P3
P9
M9
M3
M1
10
10
8
9
1
2
3
8
9
1
2
3
150k
450k
M9
M3
M1
P1
P3
P9
M9
M3
M1
P1
P3
P9
450k
150k
50k
50k
F
Figure 13. Integer Gain Difference
Amplifi ers Using Cross-Coupling
= 450k, R
V
V
LT1991
V
LT1991
V
EE
EE
S
S
GAIN = 2
GAIN = 7
IN
4
–
4
–
V
V
V
V
V
V
–
IN
IN
CC
CC
S
S
ARE SYMMETRIC: M3 AND P1.
–
+
+
+
7
7
REF
REF
G
5
5
= 150k, GAIN = 3.
OUT
OUT
–
+
10
4pF
8
9
1
2
3
450k
450k
6
6
4pF
M9
M3
M1
P1
P3
P9
V
V
GAIN = 11
OUT
OUT
V
LT1991
V
V
EE
S
V
V
V
IN
4
–
IN
IN
IN
V
+
V
–
LT1991
–
+
CC
S
+
7
REF
www.DataSheet4U.com
5
OUT
5
6
10
10
8
9
1
2
3
8
9
1
2
3
V
M9
M3
M1
P1
P3
P9
M9
M3
M1
P1
P3
P9
6
OUT
1991 F12
V
OUT
V
V
LT1991
LT1991
V
V
EE
EE
S
S
GAIN = 5
GAIN = 8
4
–
4
–
V
V
V
V
CC
CC
S
S
7
+
7
+
REF
REF
5
5
OUT
OUT
6
6
1991 F13
1991fg
V
V
OUT
OUT