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MAX4249ESD

SOT23 / Single-Supply / Low-Noise / Low-Distortion / Rail-to-Rail Op Amps

Maxim Integrated 

SOT23, Single-Supply, Low-Noise,

Low-Distortion, Rail-to-Rail Op Amps

For gains ≥10V/V, the decompensated devices

(MAX4249/MAX4255/MAX4256/MAX4257) deliver the

best distortion performance, since they have a higher

slew rate and provide a higher amount of loop gain for

a given closed-loop gain setting. Capacitive loads

below 400pF do not significantly affect distortion

results. Distortion performance remains relatively con-

stant over supply voltages.

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 = 10V/V) is

en = 15nV/√Hz. en can be reduced to 9nV/√Hz by

choosing RF = 10kΩ, RG = 1.1kΩ (AV = 10V/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 low (9nV/√Hz).

Using a Feed-Forward

Compensation Capacitor, CZ

The amplifier’s input capacitance is 11pF. 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 ≈ 11 x (RF / RG) [pF]

In the unity-gain-stable MAX4250–MAX4254, the use

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

AV = -1V/V. In the decompensated MAX4249/

MAX4255/MAX4256/MAX4257, 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 (MAX4250–MAX4254) or

greater than 5kΩ (MAX4249/MAX4255/MAX4256/

MAX4257).

RF

RG

VIN

CZ

VOUT

Figure 1. Adding Feed-Forward Compensation

100mV

AV = +2

RF = RG = 100kΩ

0mV

VIN

(50mV/

div)

VOUT

(100mV/

div)

2µs/div

Figure 2a. Pulse Response with No Feed-Forward

Compensation

100mV

AV = +2

RF = RG = 100kΩ

CZ = 11pF

VIN

0mV

VOUT

50mV/

div

100mV/

div

2µs/div

Figure 2b. Pulse Response with 10pF Feed-Forward

Compensation

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