Datasheet

INA337, INA338
11
SBOS222A
www.ti.com
INPUT AND OUTPUT VOLTAGE
The INA337 and INA338 feature nearly rail-to-rail input
behavior, with the linear input voltage range extending from
0.25V above the negative rail to 0.1V above the positive rail.
The output is able to swing to within 0.25V of the negative rail
and 0.075V of the positive rail. See Typical Characteristics
Curve “Output Swing to the Negative Rail” for additional
detail.
The INA337 uses a new, unique internal circuit topology
that provides near rail-to-rail input. Unlike other instru-
mentation amplifiers, it can linearly process inputs from
0.25V above the negative rail to 0.1V beyond the positive
rail. Conventional instrumentation amplifier circuits cannot
deliver such performance, even if rail-to-rail op amps are
used.
The ability to reject common-mode signals is derived in
most instrumentation amplifiers through a combination of
amplifier CMR and accurately matched resistor ratios. The
INA337 converts the input voltage to a current. Current-
mode signal processing provides rejection of common-
mode input voltage and power-supply variation without
accurately matched resistors.
The topology of the INA337 avoids aliasing issues that
appear in instrumentation amplifiers that use sampled
data techniques.
A simplified diagram shows the basic circuit function. The
differential input voltage, (V
IN
+) – (V
IN
–) is applied across
R
1
. The signal-generated current through R
1
comes from
A1 and A2’s output stages. A2 combines the current in R
1
with a mirrored replica of the current from A1. The result-
ing current in A2’s output and associated current mirror is
two times the current in R
1
. This current flows in (or out)
of pin 5 into R
2
. The resulting gain equation is:
G
R
R
=
2
2
1
Amplifiers A1, A2 and their associated mirrors are pow-
ered from internal charge-pumps that provide voltage
supplies that are beyond the positive negative supply. As
a result, the voltage developed on R
2
can actually swing
100mV
above the positive power-supply rail. A3 provides
a buffered output of the voltage on R
2
. A3’s input stage is
also operated from the charge-pumped power supplies for
true rail-to-rail operation.
FIGURE 4. Simplified Circuit Diagram.
INSIDE THE INA337
INPUT PROTECTION
The inputs of the INA337 are protected with internal diodes
connected to the power-supply rails. 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
0.5V, the input signal current should be limited to less than 10mA
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.
A1
V+ V–
Current Mirror
Current Mirror
I
R1
I
R1
I
R1
R
1
R
2
C
2
V
O
V
IN–
V
IN+
I
R1
2I
R1
2I
R1
2I
R1
2I
R1
2I
R1
A3
A2
IA
COMMON
0.1µF
Current Mirror
Current Mirror