INA337, INA338

11

SBOS222A

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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

with a mirrored replica of the current from A1. The result-

ing current in A2’s output and associated current mirror is

G

R

R

=

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

100mV

above the positive power-supply rail. A3 provides

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

0.1

µF

Current Mirror

Current Mirror