A combination of common-mode and differential input
voltage can cause the output of A
1
or A
2
to saturate. Figure
4 shows the output voltage swing of A
1
and A
2
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 R
1
and R
2
as shown. Resistor values greater
than 10kΩ 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 (C
1
and C
2
), either from stray capacitance or
V+
D
1
V
IN
R
1
+
–
D
2
R
2
R
G
INA111
V
O
V
IN
D
3
D
4
V+
2N4117A
1pA Leakage
Diodes:
=
FIGURE 5. Input Protection Voltage Clamp.
V
CM
–
G • V
D
2
V+
INA111
A
1
V
D
2
10kΩ
25kΩ
R
G
25kΩ
A
2
V
CM
G • V
D
2
10kΩ
10kΩ
A
3
10kΩ
G=1+
50kΩ
R
G
V
O
= G • V
D
V
D
2
V
CM
+
V–
FIGURE 4. Voltage Swing of A
1
and A
2
.
®
9
INA111
BURR-BROWN [ BURR-BROWN CORPORATION ]
