SOT23, 3V/5V, Single-Supply, Rail-to-Rail

Instrumentation Amplifiers

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13

Input Common-Mode and Output

Reference Ranges

MAX4460/MAX4461/MAX4462 have an input common-

mode range of 100mV below the negative supply to

1.7V below the positive supply.

The output reference voltage of MAX4462U/T/H is set by

REF and ranges from 100mV above the negative supply

to 1.7V below the positive supply. For maximum voltage

The output voltages of the MAX4460 and MAX4461U/

T/H are referenced to ground. Unlike the traditional

three-op-amp configuration of common instrumentation

amplifiers, the MAX4460/MAX4461/MAX4462 have

configuration) in addition to the extremely high input

impedances of MOS input differential pairs.

Input Differential Signal Range

The MAX4460/MAX4461/MAX4462 feature a proprietary

input structure optimized for small differential signals.

The unipolar output of the MAX4460/MAX4461 is nomi-

nally zero-for-zero differential input. However, these

devices are specified for inputs of 50mV to 100mV for

the unity-gain devices, 20mV to 100mV for gain of 10

devices, and 2mV to 48mV for gain of 100 devices. The

MAX4460/MAX4461 can be used with differential inputs

approaching zero, albeit with reduced accuracy.

The bipolar output of the MAX4462 allows bipolar input

ranges. The output voltage is equal to the reference

voltage for zero differential input. The MAX4462 is

specified for inputs of ±100mV for the unity gain and

gain of 10 devices, and ±20mV for gain of 100 devices.

The gain of 100 devices (MAX4462H) can be operated

beyond 20mV signal provided the reference is chosen

for unsymmetrical swing.

Output Swing

The MAX4460/MAX4461/MAX4462 are designed to

have rail-to-rail output voltage swings. However,

depending on the selected gain and supply voltage

(and output reference level of the MAX4462), the rail-to-

rail output swing is not required.

For example, consider the MAX4461U, a unity-gain

device with its ground pin as the output reference level.

The input voltage range is 0 to 100mV (50mV minimum

to meet accuracy specifications). Because the device

is unity gain and the output reference level is ground,

the output only sees excursions from ground to 100mV.

Devices with higher gain and with bipolar output such

as the MAX4462, can be configured to swing to higher

levels. In these cases, as the output approaches either

supply, accuracy may degrade, especially under heavy

output loading.

Shutdown Mode

The MAX4461U/T/H features a low-power shutdown

mode. When the SHDN pin is pulled low, the internal

transconductance and amplifier blocks are switched off

and supply current drops to typically less than 0.1µA

(Figure 1).

In shutdown, the amplifier output is high impedance.

The output transistors are turned off, but the feedback

resistor network remains connected. If the external load

is referenced to GND, the output drops to approximate-

ly GND in shutdown. The output impedance in shut-

down is typically greater than 100k

Ω. Drive SHDN high

A User Guide to Instrumentation

Amplifier Accuracy Specifications

As with any other electronic component, a complete

understanding of instrumentation amplifier specifica-

tions is essential to successfully employ these devices

in their application circuits. Most of the specifications

for these differential closed-loop gain blocks are similar

to the well-known specifications of operational ampli-

fiers. However, there are a few accuracy specifications

that could be confusing to first-time users. Therefore,

some explanations and examples may be helpful.

Accuracy specifications are measurements of close-

ness of an actual output response to its ideal

expected value. There are three main specifications

in this category:

G

Gain error

G

Gain nonlinearity error

G

Offset error

In order to understand these terms, we must look at the

transfer function of an ideal instrumentation amplifier. As

expected, this must be a straight line passing through

origin with a slope equal to the ideal gain (Figure 3). If

the ideal gain is equal to 10 and the extreme applied

input voltages are -100mV and +100mV, then the value

of the output voltages are -1V and +1V, respectively.

Note that the line passes through the origin and therefore

a zero input voltage gives a zero output response.

The transfer function of a real instrumentation amplifier

is quite different from the ideal line pictured in Figure 3.

Rather, it is a curve such as the one indicated as the

typical curve in Figure 4, connecting end points A and B.