LT1101
W U U
U
APPLICATIO S I FOR ATIO
A simple resistive offset adjust procedure is shown below.
If R = 5Ω for G = 10, and R = 50Ω for G = 100, then the
effect of R on gain error is approximately 0.006%.
Unfortunately, about 450µA has to flow through R to bias
the reference terminal (Pin 1) and to null out the worst-
case offset voltage. The total current through the resistor
network can exceed 1mA, and the micropower advantage
of the LT1101 is lost.
Gains Between 10 and 100
Gains between 10 and 100 can be achieved by connecting
two equal resistors (= Rx) between Pins 1 and 2 and
Pins 7 and 8.
Rx
Gain = 10 +
R+Rx/90
The nominal value of R is 9.2kΩ. The usefulness of this
method is limited by the fact that R is not controlled to
better than ±10% absolute accuracy in production.
However, on any specific unit, 90R can be measured
between Pins 1 and 2.
15V
10k
3
6
5
–
+
8
LT1101
OUT
10k
1
4
Input Protection
20k
Instrumentation amplifiers are often used in harsh
environments where overload conditions can occur. The
LT1101 employs PNP input transistors, consequently the
differential input voltage can be ±30V (with ±15V
supplies,±36Vwith±18Vsupplies)withoutanincreasein
inputbiascurrent.Competitiveinstrumentationamplifiers
have NPN inputs which are protected by back-to-back
diodes. When the differential input voltage exceeds ±1.3V
on these competitive devices, input current increases to
the milliampere level; more than ±10V differential voltage
can cause permanent damage.
R
10k
– 15V
LT1101 • AI01
AnotheroffsetadjustschemeusestheLT1077micropower
op amp to drive the reference Pin 1. Gain error and
common mode rejection are unaffected, the total current
increase is 45µA. The offset of the LT1077 is trimmed and
amplified to match and cancel the offset voltage of the
LT1101. Output offset null range is ±25mV.
When the LT1101’s inputs are pulled above the positive
supply, the inputs will clamp a diode voltage above the
positive supply. No damage will occur if the input current
is limited to 20mA.
1.2V TO 18V
3
6
5
–
+
5k POT
1
8
LT1101
OUT
500ΩresistorsinserieswiththeinputsprotecttheLT1101
when the inputs are pulled as much as 10V below the
negative supply.
1
8
3
+
4
7
LT1077
6
2
–
4
100k
3.3k
–1.2V TO –18V
LT1101 • AI02
1101fa
12
Linear Systems [ Linear Systems ]
