Datasheet

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 A
2
. 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 A
2
expressed in
terms of a common-mode and differential input voltages.
Output swing capability of the input amplifiers, A
1
and A
2
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
24.9kΩ
OPA177
390kΩ
390kΩ
24.9kΩ
INA115
R
G
V
O2
V
O1
V
O
FIGURE 6. Voltage Swing of A
1
and A
2
.
A
1
A
2
A
3
25kΩ25kΩ
25kΩ25kΩ
R
G
V+
V–
INA115
V
O
= G • V
D
G = 1 +
50kΩ
R
G
25kΩ
25kΩ
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