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MAX4194

Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers

Maxim Integrated 

Micropower, Single-Supply, Rail-to-Rail,

Precision Instrumentation Amplifiers

Detailed Description

Input Stage

The MAX4194–MAX4197 family of low-power instru-

mentation amplifiers implements a three-amplifier topol-

ogy (Figure 1). The input stage is composed of two

operational amplifiers that together provide a fixed-gain

differential and a unity common-mode gain. The output

stage is a conventional differential amplifier that pro-

vides an overall common-mode rejection of 115dB (G =

IN-

25kΩ

VCC

25kΩ

RG-

25kΩ

OUT

MAX4194

RG+

25kΩ

25kΩ

REF

IN+

25kΩ

VEE

Figure 1. MAX4194 Simplified Block Diagram

IN-

25kΩ

VCC

FB

25kΩ

25kΩ

RG

25kΩ

MAX4195

MAX4196

OUT

SHDN

MAX4197

25kΩ

REF

IN+

25kΩ

VEE

Figure 2. MAX4195/MAX4196/MAX4197 Simplified Block

Diagram

+10V/V). The MAX4194’s gain can be externally set

between +1V/V and +10,000V/V (Table 1). The

MAX4195/MAX4196/MAX4197 have on-chip gain-set-

ting resistors (Figure 2), and their gains are fixed at

+1V/V, +10V/V, and +100V/V, respectively.

Input Voltage Range

and Detailed Operation

The common-mode input range for all of these ampli-

fiers is VEE - 0.2V to VCC - 1.1V. Ideally, the instrumen-

tation amplifier (Figure 3) responds only to a differential

voltage applied to its inputs, IN+ and IN-. If both inputs

are at the same voltage, the output is VREF. A differen-

tial voltage at IN+ (VIN+) and IN- (VIN-) develops an

identical voltage across the gain-setting resistor, caus-

ing a current (IG) to flow. This current also flows

through the feedback resistors of the two input ampli-

fiers A1 and A2, generating a differential voltage of:

VOUT2 - VOUT1 = IG · (R1 + RG + R1)

where VOUT1 and VOUT2 are the output voltages of A1

and A2, RG is the gain-setting resistor (internal or exter-

nal to the part), and R1 is the feedback resistor of the

input amplifiers.

IG is determined by the following equation:

IG = (VIN+ - VIN-) / RG

The output voltage (VOUT) for the instrumentation

amplifier is expressed in the following equation:

VOUT = (VIN+ - VIN-) · [(2 · R1) / RG] + 1

The common-mode input range is a function of the

amplifier’s output voltage and the supply voltage. With

a power supply of VCC, the largest output signal swing

can be obtained with REF tied to VCC/2. This results in

an output voltage swing of ±VCC/2. An output voltage

swing less than full-scale increases the common-mode

input range.

VIN-

VIN+ - VIN-

VIN+

IG

RG**

IG

VOUT1

R2*

A1

R1*

VOUT2 - VOUT1

R1*

A2

VOUT2

R2*

* R1 = R2 = 25kΩ

** RG = INTERNAL TO MAX4195/MAX4196/MAX4197

R2*

RG = EXTERNAL TO MAX4194

A3

OUT

( ) VOUT = (VIN+ - VIN-) ·

1+

2R1

RG

R2*

REF

Figure 3. Instrumentation Amplifier Configuration

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