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MAX4209

Ultra-Low Offset/Drift, Precision Instrumentation Amplifiers with REF Buffer

Maxim Integrated 

Ultra-Low Offset/Drift, Precision

Instrumentation Amplifiers with REF Buffer

MAX4208

VDD

AMP

IN-

gm

gm

IN+

OUT

R2

FB

R1

REF

VDD

MAX4209

AMP

OUT

R2

IN-

gm

IN+

FB

gm

R1

REF

+1

REFIN/MODE

SHDN

G

=

1

+

R2

R1

VSS

Figure 1. MAX4208 Functional Diagram

Detailed Description

The MAX4208/MAX4209 family of instrumentation ampli-

fiers implements a patented spread-spectrum, autozero-

ing technique that minimizes the input offset error, drift

over time and temperature, and the effect of 1/f noise.

Unlike the traditional three-op amp instrumentation

amplifier, this technique allows true ground-sensing

capability combined with a low input bias current and

increased common-mode rejection.

The differential input signal is converted to a current by

an input transconductance stage. An output transcon-

ductance stage converts a portion of the output voltage

(equal to the output voltage divided by the gain) into

another precision current. These two currents are sub-

tracted and the result is fed to a loop amplifier with suffi-

cient gain to minimize errors (Figures 1 and 2). The

MAX4209 has a factory-trimmed gain of 100V/V. The

MAX4208 has an adjustable gain, set with an external

pair of resistors between OUT, FB, and REF (Figure 1).

The MAX4208/MAX4209 have an output reference input

(REF) that is connected to an external reference for bipo-

lar operation of the device. For single-supply operation,

the range for VREF is 0V to (VDD - 1.3V). Although full out-

put-swing capability and maximum symmetrical dynamic

range is obtained at REF = VDD/2, the optimal VREF set-

ting depends on the supply voltage and output-voltage

+1

REFIN/MODE

G

=

1

+

R2

R1

SHDN

VSS

Figure 2. MAX4209 Functional Diagram

swing needed by the application. The maximum recom-

mended differential input voltage is ±100mV. Linearity

and accuracy are degraded above that level. The

MAX4208/MAX4209 operate with single 2.85V to 5.5V

supply voltages or dual ±1.425V to ±2.75V supplies.

The MAX4208/MAX4209 have a shutdown feature to

reduce the supply current to 1.4µA (typ) when REFIN/

MODE is connected to VDD.

REF, REFIN/MODE, and Internal REFIN

Buffer of the MAX4208/MAX4209

In a single-supply system, bipolar operation of an

instrumentation amplifier requires the application of a

voltage reference (REF) to set the output voltage level

when a zero differential voltage is applied to the input.

The output swing is around this reference level, which

is usually set to half of the supply voltage for the largest

swing and dynamic range.

In many instrumentation amplifiers, the gain-setting

resistors as well as the RL are connected between OUT

and REF. OUT can sink and source current but the

need for REF to sink and source current is often over-

looked and can lead to significant errors. Therefore, the

MAX4208/MAX4209 include a REFIN buffer, an internal,

precision unity-gain buffer on-chip to sink and source the

currents needed at REF without loading the reference

voltage supplied at REFIN/MODE.

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