Input circuitry must provide a path for this input bias current
for proper operation. Figure 3 shows various provisions for an
input bias current path. Without a bias current path, the inputs
will float to a potential which exceeds the common-mode
range of the INA122 and the input amplifiers will saturate.
If the differential source resistance is low, the bias current
return path can be connected to one input (see the thermo-
couple example in Figure 3). With higher source impedance,
using two equal resistors provides a balanced input with
possible advantages of lower input offset voltage due to bias
current and better high-frequency common-mode rejection.
INPUT PROTECTION
The inputs of the INA122 are protected with internal diodes
connected to the power supply rails (Figure 4). These diodes
will clamp the applied signal to prevent it from damaging the
input circuitry. If the input signal voltage can exceed the
power supplies by more than 0.3V, the input signal current
should be limited to less than 5mA to protect the internal
clamp diodes. This can generally be done with a series input
resistor. Some signal sources are inherently current-limited
and do not require limiting resistors.
INPUT COMMON-MODE RANGE
The common-mode range for some common operating con-
ditions is shown in the typical performance curves. The
INA122 can operate over a wide range of power supply and
V
REF
configurations, making it impractical to provide a
comprehensive guide to common-mode range limits for all
possible conditions. The most commonly overlooked over-
load condition occurs by attempting to exceed the output
swing of A
2
, an internal circuit node that cannot be mea-
sured. Calculating the expected voltages at A
2
’s output (see
equation in Figure 4) provides a check for the most common
overload conditions.
The design of A
1
and A
2
are identical and their outputs can
swing to within approximately 100mV of the power supply
rails, depending on load conditions. When A
2
’s output is
saturated, A
1
can still be in linear operation, responding to
changes in the non-inverting input voltage. This may give the
appearance of linear operation but the output voltage is invalid.
Microphone,
Hydrophone
etc.
INA122
47kΩ
47kΩ
Thermocouple
INA122
10kΩ
INA122
Center-tap provides
bias current return.
A single supply instrumentation amplifier has special design
considerations. Using commonly available single-supply op
amps to implement the two-op amp topology will not yield
equivalent performance. For example, consider the condition
where both inputs of common single-supply op amps are
FIGURE 3. Providing an Input Common-Mode Current Path.
V+
+
V
IN
+ 0.5V
+
V
IN
(3)
(8)
A
1
100kΩ
25kΩ
R
G
25kΩ
(1)
A
2
–
V
IN
(2)
–
V
IN
+ 0.5V
100kΩ
Ref
V
02
+
–
– 25kΩ + 0.6V
V
O2
= 1.25V
IN
– (V
IN
– V
IN
)
R
G
(Voltages are referred to V
REF
)
(V–) + 0.1V
≤
V
02
≤
(V+) –0.1V
V
O
V–
V+
V–
FIGURE 4. INA122 Simplified Circuit Diagram.
®
INA122
8
BURR-BROWN [ BURR-BROWN CORPORATION ]
