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

±13Vonthesecompetitivedevices,

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

7 and 8.

The nominal value of R is 1.84k

Ω. 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

Gain = 10 +

FREQUENCY (Hz)

1

0

20

40

60

80

120

10

100

1k

10k

100k

1M

100

LT1102 • AI01

G = 100

18pF PIN 1

TO PIN 2

G = 10

G = 100