®

INA111

9

A combination of common-mode and differential input

voltage can cause the output of A1 or A2 to saturate. Figure

4 shows the output voltage swing of A1 and A2 expressed in

terms of a common-mode and differential input voltages.

For applications where input common-mode range must be

maximized, limit the output voltage swing by connecting the

INA111 in a lower gain (see performance curve “Input

Common-Mode Voltage Range vs Output Voltage”). If

necessary, add gain after the INA111 to increase the voltage

swing.

Input-overload often produces an output voltage that appears

normal. For example, consider an input voltage of +14V on

one input and +15V 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

INA111 will be near 0V even though both inputs are

overloaded.

INPUT PROTECTION

Inputs of the INA111 are protected for input voltages from

0.7V below the negative supply to 15V above the positive

power supply voltages. If the input current is limited to less

than 1mA, clamp diodes are not required; internal junctions

will clamp the input voltage to safe levels. If the input source

can supply more than 1mA, use external clamp diodes as

shown in Figure 5. The source current can be limited with

series resistors R1 and R2 as shown. Resistor values greater

than 10k

Ω will contribute noise to the circuit.

A diode formed with a 2N4117A transistor as shown in

Figure 5 assures low leakage. Common signal diodes such as

the 1N4148 may have leakage currents far greater than the

input bias current of the INA111 and are usually sensitive to

light.

INPUT FILTERING

The INA111’s FET input allows use of an R/C input filter

without creating large offsets due to input bias current.

Figure 6 shows proper implementation of this input filter to

preserve the INA111’s excellent high frequency common-

mode rejection. Mismatch of the common-mode input ca-

pacitance (C1 and C2), either from stray capacitance or

FIGURE 5. Input Protection Voltage Clamp.

FIGURE 4. Voltage Swing of A

1 and A2.

INA111

V

O

V+

V+

V

IN

–

V

IN

+

D

1

D

3

D

2

D

4

Diodes:

=

2N4117A

1pA Leakage

R

G

R

1

R

2

A

1

A

2

A

3

10k

Ω

10k

Ω

10k

Ω

10k

Ω

R

G

V+

V–

INA111

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