11

®

INA115

FIGURE 7. ECG Amplifier with Right Leg Drive.

INPUT COMMON-MODE RANGE

The linear common-mode range of the input op amps of the

INA115 is approximately

±13.75V (or 1.25V from the power

supplies). As the output voltage increases, however, the linear

input range will be limited by the output voltage swing of the

input amplifiers, A

1 and A2. The common-mode range is

related to the output voltage of the complete amplifier—see

performance curve “Input Common-Mode Range vs Output

Voltage.”

A combination of common-mode and differential input sig-

nals can cause the output of A

1

or A

2

to saturate. Figure 6

shows the output voltage swing of A

1 and A2 expressed in

terms of a common-mode and differential input voltages.

Output swing capability of the input amplifiers, A

1 and A2 is

the same as the output amplifier, A

3

. For applications where

input common-mode range must be maximized, limit the

output voltage swing by connecting the INA115 in a lower

gain (see performance curve “Input Common-Mode Voltage

Range vs Output Voltage”). If necessary, add gain after the

INA115 to increase the voltage swing.

Input-overload often produces an output voltage that appears

normal. For example, an input voltage of +20V on one input

and +40V on the other input will obviously exceed the linear

common-mode range of both input amplifiers. Since both

input amplifiers are saturated to the nearly the same output

voltage limit, the difference voltage measured by the output

amplifier will be near zero. The output of the INA115 will be

near 0V even though both inputs are overloaded.

INPUT PROTECTION

The inputs of the INA115 are individually protected for

voltages up to

±40V. For example, a condition of –40V on one

input and +40V on the other input will not cause damage.

Internal circuitry on each input provides low series impedance

under normal signal conditions. To provide equivalent protec-

tion, series input resistors would contribute excessive noise. If

the input is overloaded, the protection circuitry limits the input

current to a safe value (approximately 1.5mA). The typical

performance curve “Input Bias Current vs Common-Mode

Input Voltage” shows this input current limit behavior. The

inputs are protected even if the power supply voltage is zero.

OTHER APPLICATIONS

See the INA114 data sheet for other applications circuits of

general interest.

LA

RL

RA

24.9k

Ω

OPA177

390k

Ω

390k

Ω

24.9k

Ω

INA115

R

G

V

O2

V

O1

V

O

FIGURE 6. Voltage Swing of A

1 and A2.

A

1

A

2

A

3

25k

Ω

25k

Ω

25k

Ω

25k

Ω

R

G

V+

V–

INA115

V

O = G • VD

G = 1 +

50k

Ω

R

G

25k

Ω

25k

Ω

V

CM –

G • V

D

2

V

D

2

V

D

2

V

CM

V

CM +

G • V

D

2

Over-Voltage

Protection

Over-Voltage

Protection