INA337 datasheet(12/28 Pages) TI2

12 / 28 page

INA326, INA327

SBOS222D

www.ti.com

The INA326 uses a new, unique internal circuit topology

that provides true rail-to-rail input. Unlike other instrumen-

tation amplifiers, it can linearly process inputs up to 20mV

below the negative power-supply rail, and 100mV above

the positive power-supply rail. Conventional instrumenta-

tion 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 INA326 converts the input voltage to a current.

Current-mode signal processing provides rejection of com-

mon-mode input voltage and power-supply variation with-

out accurately matched resistors.

A simplified diagram shows the basic circuit function. The

differential input voltage, (VIN+) – (VIN–) is applied across

R1. The signal-generated current through R1 comes from

A1 and A2’s output stages. A2 combines the current in R1

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 R1. This current flows in (or out)

of pin 5 into R2. The resulting gain equation is:

= 2 2

Amplifiers A1, A2, and their associated mirrors are pow-

ered from internal charge-pumps that provide voltage

supplies that are beyond the positive and negative supply

rails. As a result, the voltage developed on R2 can actually

swing 20mV

below the negative power-supply rail, and

100mV

above the positive supply rail. A3 provides a

buffered output of the voltage on R2. A3’s input stage is

also operated from the charge-pumped power supplies for

true rail-to-rail operation.

FIGURE 5. Simplified Circuit Diagram.

INSIDE THE INA326

V+

−

Current Mirror

−

IN+

COMMON

0.1

µF

Current Mirror

INA326

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