LT1991 Datasheet, PDF (15/28 Pages) Linear Integrated Systems – Precision, 100UA Gain Selectable Amplifier

LT1991
APPLICATIONS INFORMATION
Attenuation Using the P Input Resistors
Attenuation happens as a matter of fact in difference
amplifier configurations, 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.
V_IN
R_A
V_INT
V_IN
OKAY UP
TO ±60V
R_{G VINT = A • VIN}
A = R_G/(R_A + R_G)
1 ^450k
2 ^150k
V_INT
+
4pF
3 ^50k
450k
LT1991
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CLASSICAL ATTENUATOR
LT1991 ATTENUATING TO THE +INPUT BY
DRIVING AND GROUNDING AND FLOATING
INPUTS R_A = 450k, R_G = 50k, SO A = 0.1.
1991 F06
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%.
100
10
1
Table 2. Configuring the P Pins for Various Attenuations.
Those Shown in Bold Are Functional Even When the Input Drive
Exceeds the Supplies.
P9, P3, P1, REF Connection
A
P9
P3
P1
REF
0.0714
Ground
Ground
Drive
Ground
0.0769
Ground
Ground
Drive
Float
0.0909
Ground
Float
Drive
Ground
0.1
Ground
Float
Drive
Float
0.143
Ground
Ground
Drive
Drive
0.182
Ground
Float
Drive
Drive
0.2
Float
Ground
Drive
Ground
0.214
Ground
Drive
Ground
Ground
0.231
Ground
Drive
Float
Ground
0.25
Float
Ground
Drive
Float
0.286
Ground
Drive
Drive
Ground
0.308
Ground
Drive
Drive
Float
0.357
Ground
Drive
Drive
Drive
0.4
Float
Ground
Drive
Drive
0.5
Float
Float
Drive
Ground
0.6
Float
Drive
Ground
Ground
0.643
Drive
Ground
Ground
Ground
0.692
Drive
Ground
Float
Ground
0.714
Drive
Ground
Drive
Ground
0.75
Float
Drive
Float
Ground
0.769
Drive
Ground
Drive
Float
0.786
Drive
Ground
Drive
Drive
0.8
Float
Drive
Drive
Ground
0.818
Drive
Float
Ground
Ground
0.857
Drive
Drive
Ground
Ground
0.9
Drive
Float
Float
Ground
0.909
Drive
Float
Drive
Ground
0.923
Drive
Drive
Float
Ground
0.929
Drive
Drive
Drive
Ground
1
Drive
Drive
Drive
Drive
0.1
0.01
0
50 100 150 200 250 300 350
COUNT
1991 F07
Figure 7. Over 346 Unique Gain Settings Achievable with the
LT1991 by Combining Attenuation with Noninverting Gain
1991fg
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