equal to 0V. The outputs of both A
1
and A
2
must be 0V. But
any small positive voltage applied to V
IN+
requires that A
2
’s
output must swing below 0V, which is clearly impossible
without a negative power supply.
To achieve common-mode range that extends to single-
supply ground, the INA122 uses precision level-shifting
buffers on its inputs. This shifts both inputs by approxi-
mately +0.5V, and through the feedback network, shifts A
2
’s
output by approximately +0.6V. With both inputs and V
REF
at single-supply, A
2
’s output is well within its linear range.
A positive V
IN+
causes A
2
’s output to swing below 0.6V.
As a result of this input level-shifting, the voltages at pin 1
and pin 8 are not equal to their respective input terminal
voltages (pins 2 and 3). For most applications, this is not
important since only the gain-setting resistor connects to
these pins.
LOW VOLTAGE OPERATION
The INA122 can be operated on a single power supply as
low as +2.2V (or a total of +2.2V on dual supplies). Perfor-
mance remains excellent throughout the power supply range
up to +36V (or
±18V).
Most parameters vary only slightly
throughout this supply voltage range—see typical perfor-
mance curves.
Operation at very low supply voltage requires careful atten-
tion to ensure that the common-mode voltage remains within
its linear range.
LOW QUIESCENT CURRENT OPERATION
The INA122 maintains its low quiescent current (60µA)
while the output is within linear operation (up to 200mV
from the supply rails). When the input creates a condition
that overdrives the output into saturation, quiescent current
increases. With V
O
overdriven into the positive rail, the
quiescent current increases to approximately 400µA. Like-
wise, with V
O
overdriven into the negative rail (single
supply ground) the quiescent current increases to approxi-
mately 200µA.
OUTPUT CURRENT RANGE
Output sourcing and sinking current values versus the output
voltage ranges are shown in the typical performance curves.
The positive and negative current limits are not equal.
Positive output current sourcing will drive moderate to high
load impedances. Battery operation normally requires the
careful management of power consumption to keep load
impedances very high throughout the design.
+5V
REF200
200µA
≈
200mV
+
V
IN
R
G
1
V–
IN
(60µA)
V
CM
≈
100mV
1kΩ
3
8
7
INA122
4
6
5
Ref
(1)
V
O
= 0.1V to 4.9V
V
O
NOTE: (1) To accomodate bipolar input signals,
V
REF
can be offset to a positive voltage. Output
voltage is then referred to the voltage applied to Ref.
2
FIGURE 5. Micropower Single Supply Bridge Amplifier.
V+
+5V
Load
8
I
L
2.5A
+
V
IN
Shunt
R
S
0.02Ω
3
8
50mV
R
G
1
V–
IN
1
7
INA122
5
4
G = 100
6
1kΩ
0.47µF
2
V
REF
D
+IN
ADS7816
12-Bit A/D
CS
CLK
5
7
Chip Select
Clock
6
Serial Data
2
3
–IN
Differential measurement
avoids ground loop errors.
4
FIGURE 6. Single-Supply Current Shunt Measurement.
®
9
INA122
BURR-BROWN [ BURR-BROWN CORPORATION ]
