LT1991 Linear Technology, LT1991 Datasheet - Page 18

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

APPLICATIONS INFORMATION
Table 4. Connections Giving Difference Gains for the LT1991
LT1991
Difference Amplifi ers
The resistors in the LT1991 allow it to easily make differ-
ence amplifi ers also. Figure 10 shows the basic 4-resistor
difference amplifi er and the LT1991. A difference gain of
3 is shown, but notice the effect of the additional dashed
connections. By connecting the 450k resistors in paral-
lel, the gain is reduced by a factor of 2. Of course, with
so many resistors, there are many possible gains. Table
4 shows the difference gains and how they are achieved.
Note that, as for inverting amplifi ers, the noise gain is 1
more than the signal gain.
18
0.077
0.273
Gain
0.25
2.25
0.1
0.3
0.4
1.5
1.8
2.5
4.5
10
12
13
1
3
4
6
9
P1, P3, P9
P1, P3
P1, P9
P1, P3
P3, P9
P1, P9
P3, P9
V
P1
P1
P1
P3
P3
P1
P3
P9
P9
P3
P9
P9
IN
+
M1, M3, M9
M1, M3
M1, M9
M1, M3
M3, M9
M1, M9
M3, M9
V
M1
M1
M1
M3
M3
M1
M3
M9
M9
M3
M9
M9
IN
–
M3, M9
M1, M9
M1, M3
Output
M9
M3
M9
M9
M1
M3
M3
M1
M1
GND (REF)
P3, P9
P1, P9
P1, P3
P9
P3
P9
P9
P1
P3
P3
P1
P1
TO CHANGE
PARALLEL
Figure 10. Difference Amplifi er Using the LT1991. Gain Is Set
Simply by Connecting the Correct Resistors or Combinations
of Resistors. Gain of 3 Is Shown, with Dashed Lines Modifying
It to Gain of 1.5. Noise Gain Is Optimal.
R
F
, R
V
V
IN
G
IN
–
+
CLASSICAL DIFFERENCE AMPLIFIER IMPLEMENTED
WITH LT1991. R
ADDING THE DASHED CONNECTIONS CONNECTS THE
TWO 450k RESISTORS IN PARALLEL, SO R
TO 225k. GAIN BECOMES 225k/150k = 1.5.
V
V
CLASSICAL DIFFERENCE AMPLIFIER USING THE LT1991
IN
IN
10
8
9
1
2
3
–
+
P1
P3
P9
M9
M3
M1
R
R
150k
450k
450k
150k
50k
G
G
50k
F
= 450k, R
–
+
G
= 150k, GAIN = 3.
R
R
–
+
F
F
4pF
www.DataSheet4U.com
V
GAIN = R
450k
450k
OUT
4pF
= GAIN • (V
F
F
/R
IS REDUCED
LT1991
G
V
OUT
IN
+
– V
6
5
IN
–
)
V
1991 F10
1991fg
OUT