LT1996 Linear Technology, LT1996 Datasheet - Page 18

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

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APPLICATIO S I FOR ATIO
LT1996
Difference Amplifier: Additional Integer Gains Using
Cross-Coupling
Figure 12 shows the basic difference amplifier as well as
the LT1996 in a difference gain of 27. But notice the effect
of the additional dashed connections. This is referred to as
“cross-coupling” and has the effect of reducing the differ-
ential gain from 27 to 18. Using this method, additional
integer gains are achievable, as shown in Table 5 below.
Note that the equations can be written by inspection from
the V
simply the opposite (swap P for M and M for P). The
method is the same as for the LT1991, except that the
LT1996 applies a multiplier of 9. 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.
Table 5. Connections Using Cross-Coupling. Note That Equations
Can Be Written by Inspection of the V
Gain
18
18
45 P81, M27, M9 M81, P27, P9 81 – 27 – 9 117
54
63 P81, P9, M27 M81, M9, P27 81 + 9 – 27 117
72
99 P81, P27, M9 M81, M27, P9 81 + 27 – 9 117
4
IN
P81, M27
P27, M9
P81, M9
+
V
IN
connections, and that the V
+
M81, P27
M27, P9
M81, P9
V
V
V
IN
IN
IN
U
–
+
–
Figure 12. Another Method of Selecting Difference Gain Is “Cross-Coupling.”
The Additional Method Means the LT1996 Provides Extra Integer Gains
R
R
CLASSICAL DIFFERENCE AMPLIFIER
G
G
Equation Gain
U
81 – 27
27 – 9
81 – 9
Gain
–
+
IN
Noise –3dB BW R
R
R
108
+
F
F
39
90
Column
W
V
GAIN = R
OUT
IN
–
= GAIN • (V
kHz
14
connections are
5
5
5
6
5
F
/R
G
V
OUT
IN
Typ kΩ Typ kΩ
+
– V
U
46
12
16
16
45
45
IN
IN
+
–
)
R
16
IN
6
6
5
6
4
–
COUPLING
V
V
IN
CROSS-
IN
–
+
V
V
V
V
V
V
IN
IN
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 P27 AND M9 GIVES 27 – 9 = 18.
8
9
1
2
3
10
10
10
8
9
1
2
3
8
9
1
2
3
8
9
1
2
3
450k/81
450k/27
450k/27
450k/81
450k/9
450k/9
M81
M27
M9
P9
P27
P81
M81
M27
M9
P9
P27
P81
M81
M27
M9
P9
P27
P81
F
= 450k, R
V
V
V
LT1996
LT1996
LT1996
IN
V
V
V
EE
GAIN = 18
EE
GAIN = 45
EE
GAIN = 72
S
S
S –
–
–
4
4
–
4
Figure 13. Integer Gain Difference
Amplifiers Using Cross-Coupling
ARE SYMMETRIC: M27 AND P9.
V
V
V
V
V
V
CC
CC
CC
S
S
S
+
G
7
7
+
7
+
REF
REF
REF
= 16.7k, GAIN = 27.
5
5
5
–
+
OUT
OUT
OUT
450k
450k
4pF
4pF
4pF
4pF
6
6
6
V
V
V
OUT
OUT
OUT
LT1996
V
V
V
V
V
V
IN
IN
IN
IN
IN
IN
+
–
+
–
–
+
6
5
V
OUT
10
10
10
8
9
1
2
3
8
9
1
2
3
8
9
1
2
3
1996 F10
M81
M27
M9
P9
P27
P81
M81
M27
M9
P9
P27
P81
M81
M27
M9
P9
P27
P81
GAIN = 99
V
V
V
LT1996
LT1996
V
LT1996
V
V
EE
GAIN = 54
EE
GAIN = 63
EE
S
S
S –
4
–
4
–
4
V
V
V
V
V
V
CC
CC
CC
S
S
S
+
+
7
7
7
+
REF
REF
REF
5
5
5
OUT
OUT
OUT
6
6
6
1996f
1996 F13
V
V
OUT
OUT