LT1102
8
1102fb
In the two op amp instrumentation amplifier configura-
tion, the first amplifier is basically in unity gain, and the
second amplifier provides all the voltage gain. In the
LT1102, the second amplifier is decompensated for gain
of 10 stability, therefore high slew rate and bandwidth are
achieved. Common mode rejection versus frequency
is also optimized in the G = 10 mode, because the
bandwidths of the two op amps are similar. When G = 100,
this statement is no longer true; however, by connecting
an 18pF capacitor between pins 1 and 2, a common mode
AC gain is created to cancel the inherent roll-off. From
200Hz to 30kHz, CMRR versus frequency is improved by
an order of magnitude.
Input Protection
Instrumentation amplifiers are often used in harsh
environments where overload conditions can occur.
The LT1102 employs FET input transistors, consequently
the differential input voltage can be ±30V (with ±15V
supplies, ±36V with ±18V supplies). Some competitive
instrumentation amplifiers have NPN inputs which are
protected by back-to-back diodes. When the differential
input Voltage exceeds ±13V on these competitive devices,
input current increases to milliampere level; more than
±10V differential voltage can cause permanent damage.
When the LT1102 inputs are pulled below the negative
supply or above the positive supply, the inputs will clamp
a diode voltage below or above the supplies. No damage
will occur if the input current is limited to 20mA.
Common Mode Rejection Ratio
vs Frequency
Gains Between 10 and 100
Gains between 10 and 100 can be achieved by connecting
two equal resistors (= R
X
) between pins 1 and 2 and pins
7 and 8.
The nominal value of R is 1.84kΩ. 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.
APPLICATIO S I FOR ATIO
WUUU
Gain = 10 +
R
X
R + R
X
/90
FREQUENCY (Hz)
1
0
COMMON MODE REJECTION RATIO (dB)
20
40
60
80
120
10
100 1k 10k
100k 1M
100
LT1102 • AI01
V
S
= ±15V
T
A
= 25°C
G = 100
18pF PIN 1
TO PIN 2
G = 10
G = 100
