MAX4477_12 datasheet(10/21 Pages) MAXIM

Part #	MAX4477

Description	SOT23, Low-Noise, Low-Distortion, Wide-Band, Rail-to-Rail Op Amps

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Manufacturer	MAXIM [Maxim Integrated Products]

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MAX4475–MAX4478/MAX4488/MAX4489

SOT23, Low-Noise, Low-Distortion,

Wide-Band, Rail-to-Rail Op Amps

10

Maxim Integrated

Low Noise

The amplifier’s input-referred noise-voltage density is

dominated by flicker noise at lower frequencies, and by

thermal noise at higher frequencies. Because the ther-

mal noise contribution is affected by the parallel combi-

nation of the feedback resistive network (RF || RG,

Figure 1), these resistors should be reduced in cases

where the system bandwidth is large and thermal noise

is dominant. This noise contribution factor decreases,

however, with increasing gain settings.

For example, the input noise-voltage density of the cir-

cuit with RF = 100kΩ, RG = 11kΩ (AV = +5V/V) is

en = 14nV/√Hz, en can be reduced to 6nV/√Hz by

choosing RF = 10kΩ, RG = 1.1kΩ (AV = +5V/V), at the

expense of greater current consumption and potentially

higher distortion. For a gain of 100V/V with RF = 100kΩ,

RG = 1.1kΩ, the en is still a low 6nV/√Hz.

Using a Feed-Forward Compensation

Capacitor, CZ

The amplifier’s input capacitance is 10pF. If the resis-

tance seen by the inverting input is large (feedback

network), this can introduce a pole within the amplifier’s

bandwidth resulting in reduced phase margin.

Compensate the reduced phase margin by introducing

a feed-forward capacitor (CZ) between the inverting

input and the output (Figure 1). This effectively cancels

the pole from the inverting input of the amplifier.

Choose the value of CZ as follows:

CZ = 10 x (RF / RG) [pF]

In the unity-gain stable MAX4475–MAX4478, the use of

a proper CZ is most important for AV = +2V/V, and

AV = -1V/V. In the decompensated MAX4488/

MAX4489, CZ is most important for AV = +10V/V.

Figures 2a and 2b show transient response both with

and without CZ.

Using a slightly smaller CZ than suggested by the for-

mula above achieves a higher bandwidth at the

expense of reduced phase and gain margin. As a gen-

eral guideline, consider using CZ for cases where RG ||

RF is greater than 20kΩ (MAX4475–MAX4478) or

greater than 5k

Ω (MAX4488/MAX4489).

Applications Information

The MAX4475–MAX4478/MAX4488/MAX4489 combine

good driving capability with ground-sensing input and

rail-to-rail output operation. With their low distortion and

low noise, they are ideal for use in ADC buffers, med-

ical instrumentation systems and other noise-sensitive

applications.

Ground-Sensing and Rail-to-Rail Outputs

The common-mode input range of these devices

extends below ground, and offers excellent common-

mode rejection. These devices are guaranteed not to

undergo phase reversal when the input is overdriven

(Figure 3).

Figure 4 showcases the true rail-to-rail output operation

of the amplifier, configured with AV = 5V/V. The output

swings to within 8mV of the supplies with a 10k

Ω load,

making the devices ideal in low-supply voltage applica-

tions.

Power Supplies and Layout

The MAX4475–MAX4478/MAX4488/MAX4489 operate

from a single +2.7V to +5.5V power supply or from dual

supplies of ±1.35V to ±2.75V. For single-supply opera-

tion, bypass the power supply with a 0.1µF ceramic

VOUT

2V/div

VIN

2V/div

0V

AV = +1

VDD = +5V

RL = 10k

Ω

40

µs/div

VOUT

1V/div

5V

0V

20

µs/div

Figure 3. Overdriven Input Showing No Phase Reversal

Figure 4. Rail-to-Rail Output Operation

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