LT1996
13
1996f
Figure 6. LT1996 Provides for Easy Attenuation to the Op Amp’s
+Input. The P9 Input Can Be Taken Well Outside of the Supplies
Figure 7. Over 600 Unique Gain Settings Achievable with the
LT1996 by Combining Attenuation with Noninverting Gain
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 LT1996 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 700 unique gains between
0.0085 and 118, as plotted in Figure 7. This is too large a
number to tabulate, but the designer can calculate achiev-
able 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 worst case steps of
about 50% as seen in Figure 7.
Table 2. Configuring the P Pins for Various Attenuations. Those
Shown in Bold Are Functional Even When the Input Drive
Exceeds the Supplies
P81, P27, P9, REF Connection
A P81 P27 P9 REF
0.0085 Grounded Grounded Grounded Driven
0.0092 Grounded Grounded Float Driven
0.0110 Grounded Float Grounded Driven
0.0122 Grounded Float Float Driven
0.0270 Float Grounded Grounded Driven
0.0357 Float Grounded Float Driven
0.0763 Grounded Grounded Driven Grounded
0.0769 Grounded Grounded Driven Float
0.0847 Grounded Grounded Driven Driven
0.0989 Grounded Float Driven Grounded
0.1 Grounded Float Driven Float
0.110 Grounded Float Driven Driven
0.229 Grounded Driven Grounded Grounded
0.231 Grounded Driven Grounded Float
0.237 Grounded Driven Grounded Driven
0.243 Float Grounded Driven Grounded
0.248 Grounded Driven Float Grounded
0.25 Float Grounded Driven Float
0.25 Grounded Driven Float Float
0.257 Grounded Driven Float Driven
0.270 Float Grounded Driven Driven
0.305 Grounded Driven Driven Grounded
0.308 Grounded Driven Driven Float
0.314 Grounded Driven Driven Driven
0.686 Driven Grounded Grounded Grounded
0.692 Driven Grounded Grounded Float
0.695 Driven Grounded Grounded Driven
0.730 Float Driven Grounded Grounded
0.743 Driven Grounded Float Grounded
0.75 Float Driven Grounded Float
0.752 Driven Grounded Float Driven
0.757 Float Driven Grounded Driven
0.763 Driven Grounded Driven Grounded
0.769 Driven Grounded Driven Float
0.771 Driven Grounded Driven Driven
0.890 Driven Float Grounded Grounded
0.9 Float Float Driven Grounded
0.901 Driven Float Grounded Driven
0.915 Driven Driven Grounded Grounded
0.923 Driven Driven Grounded Float
0.924 Driven Driven Grounded Driven
0.964 Float Driven Float Grounded
0.973 Float Driven Driven Grounded
0.988 Driven Float Float Grounded
0.989 Driven Float Driven Grounded
0.991 Driven Driven Float Grounded
0.992 Driven Driven Driven Grounded
APPLICATIO S I FOR ATIO
WUU
U
V
IN
OKAY UP
TO ±60V
+
450k/81
450k/27
450k/9
450k
5
1
2
3
LT1991 ATTENUATING TO THE +INPUT BY
DRIVING AND GROUNDING AND FLOATING
INPUTS R
A
= 50k, R
G
= 50k/9, SO A = 0.1.
V
INT
V
INT
V
INT
= A • V
IN
A = R
G
/(R
A
+ R
G
)
V
IN
LT1996
R
A
R
G
1996 F06
CLASSICAL ATTENUATOR
COUNT
0
1000
100
10
1
0.1
0.01
0.001
300
1996 F07
100 200 400
500 700
600
GAIN
