Datasheet

LT1991
15
1991fh
V
IN
OKAY UP
TO ±60V
+
50k
150k
450k
450k
5
1
2
3
LT1991 ATTENUATING TO THE +INPUT BY
DRIVING AND GROUNDING AND FLOATING
INPUTS R
A
= 450k, R
G
= 50k, SO A = 0.1.
V
INT
V
INT
V
INT
= A • V
IN
A = R
G
/(R
A
+ R
G
)
V
IN
LT1991
R
A
R
G
1991 F06
CLASSICAL ATTENUATOR
4pF
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.
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%.
Figure 6. LT1991 Provides for Easy Attenuation to the Op Amp’s
+Input. The P1 Input Can Be Taken Well Outside of the Supplies
Figure 7. Over 346 Unique Gain Settings Achievable with the
LT1991 by Combining Attenuation with Noninverting Gain
COUNT
0
100
10
1
0.1
0.01
150
1991 F07
50 100 200
250 350
300
GAIN
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
APPLICATIONS INFORMATION