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

Instrumentation Amplifiers

16

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Applications Information

Setting the Gain (MAX4460)

The MAX4460 gain is set by connecting a resistive-

divider from OUT to GND, with the center tap connect-

ed to FB (Figure 2). The gain is calculated by:

Gain = 1 + R2 / R1

Because FB has less than 100pA IB, high-valued resis-

tors can be used without significantly affecting the gain

accuracy. The sum of resistors (R1 + R2) near 100k

Ω is

a good compromise. Resistor accuracy directly affects

gain accuracy. Resistor sum less than 20k

Ω should not

be used because their loading can slightly affect output

accuracy.

Capacitive-Load Stability

The MAX4460/MAX4461/MAX4462 are capable of dri-

ving capacitive loads up to 100pF.

Applications needing higher capacitive drive capability

may use an isolation resistor between OUT and the

load to reduce ringing on the output signal. However

this reduces the gain accuracy due to the voltage drop

across the isolation resistor.

Output Loading

For best performance, the output loading should be to

the potential seen at REF for the MAX4462 or to ground

for the MAX4460/MAX4461.

REF Input (MAX4462)

The REF input of the MAX4462 can be connected to any

voltage-divider with sink and source capability works

resistive dividers should be avoided because the 100k

Ω

(typ) input impedance of REF causes amplitude-depen-

dent variations in the divider’s output.

Bandgap references, either series or shunt, can be

used to drive REF. This provides a voltage and temper-

ature invariant reference. This same reference voltage

can be used to bias bridge sensors to eliminate supply

voltage ratiometricity. For proper operation, the refer-

ence must be able to sink and source at least 25µA.

In many applications, the MAX4462 is connected to a

CODEC or other device with a reference voltage out-

put. In this case, the receiving device’s reference out-

put makes an ideal reference voltage. Verify the

reference output of the device is capable of driving the

MAX4462’s REF input.

Power-Supply Bypass and Layout

Good layout technique optimizes performance by

decreasing the amount of stray capacitance at the

instrumentation amplifier’s gain-setting pins. Excess

capacitance produces peaking in the amplifier’s fre-

quency response. To decrease stray capacitance, min-

imize trace lengths by placing external components as

close to the instrumentation amplifier as possible. For

best performance, bypass each power supply to

ground with a separate 0.1µF capacitor.

Microphone Amplifier

The MAX4462’s bipolar output, along with its excellent

common-mode rejection ratio, makes it suitable for pre-

cision microphone amplifier applications. Figure 7 illus-

trates one such circuit. In this case, the electret

microphone is resistively biased to the supply voltage

through a 2.2k

Ω pullup resistor. The MAX4462 directly

senses the output voltage at its noninverting input, and

indirectly senses the microphone’s ground through an

AC-coupling capacitor. This technique provides excel-

lent rejection of common-mode noise picked up by the

microphone lead wires. Furthermore, ground noise from

distantly located microphones is reduced.

The single-ended output of the MAX4462 is converted to

differential through a single op amp, the MAX4335. The

op amp forces the midpoint between OUT+ and OUT- to

be equal to the reference voltage. The configuration

does not change the MAX4662T’s fixed gain of 10.