LT1991 Linear Technology, LT1991 Datasheet - Page 15

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

APPLICATIONS INFORMATION
Attenuation Using the P Input Resistors
Attenuation happens as a matter of fact in difference
amplifi er confi gurations, but it is also used for reducing
peak signal level or improving input common mode range
even in single ended systems. When signal conditioning
indicates a need for attenuation, the LT1991 resistors are
ready at hand. The four precision resistors can provide
several attenuation levels, and these are tabulated in
Table 2 as a design reference.
Figure 6. LT1991 Provides for Easy Attenuation to the Op Amp’s
+Input. The P1 Input Can Be Taken Well Outside of the Supplies.
Because the attenuations and the noninverting gains are set
independently, they can be combined. This provides high
gain resolution, about 340 unique gains between 0.077
and 14, as plotted in Figure 7. This is too large a number
to tabulate, but the designer can calculate achievable gain
by taking the vector product of the gains and attenuations
in Tables 1 and 2, and seeking the best match. Average
gain resolution is 1.5%, with a worst-case of 7%.
V
Figure 7. Over 346 Unique Gain Settings Achievable with the
LT1991 by Combining Attenuation with Noninverting Gain
CLASSICAL ATTENUATOR
IN
R
R
A
G
V
INT
V
A = R
0.01
100
INT
0.1
10
1
= A • V
G
0
/(R
OKAY UP
TO ±60V
A
IN
+ R
50
V
G
IN
)
100
150
1
2
3
COUNT
LT1991 ATTENUATING TO THE +INPUT BY
DRIVING AND GROUNDING AND FLOATING
INPUTS R
200
450k
150k
50k
A
250
= 450k, R
V
INT
300
1991 F07
G
450k
+
= 50k, SO A = 0.1.
350
4pF
5
LT1991
1991 F06
Table 2. Confi guring the P Pins for Various Attenuations.
Those Shown in Bold Are Functional Even When the Input Drive
Exceeds the Supplies.
0.0714
0.0769
0.0909
0.143
0.182
0.214
0.231
0.286
0.308
0.357
0.643
0.692
0.714
0.769
0.786
0.818
0.857
0.909
0.923
0.929
0.25
0.75
0.1
0.2
0.5
0.4
0.6
0.8
0.9
A
1
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Float
Float
Float
Float
Float
Float
Float
P9
P9, P3, P1, REF Connection
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Float
Float
Float
Float
Float
Float
Float
P3
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Ground
Ground
Ground
Ground
Ground
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Drive
Float
Float
Float
Float
Float
P1
LT1991
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Drive
Drive
Drive
Drive
Drive
Drive
15
Float
Float
Float
Float
Float
REF
1991fg