Shutdown Mode

The MAX9727 features a low-power shutdown mode

that reduces quiescent current consumption to less

than 0.1µA and extends battery life for portable appli-

cations. Drive SHDN low to disable the amplifiers and

the charge pump. In shutdown mode, each amplifier’s

output resistance is high impedance to small signals.

The resulting output resistance seen by the load is

determined by the series combination of the amplifier’s

external gain-setting resistors in parallel with the ampli-

fier’s shutdown output resistance.

Click-and-Pop Suppression

In conventional single-supply audio amplifiers, the out-

put-coupling capacitor contributes significantly to audi-

ble clicks and pops. Upon startup, the amplifier charges

the coupling capacitor to its bias voltage, typically half

the supply. Likewise, on shutdown, the capacitor is dis-

charged. This results in a DC shift across the capacitor,

which appears as an audible transient at the speaker

connected to the output of the power amplifier of the

audio system. Since the MAX9727 does not require out-

put-coupling capacitors, this problem does not arise.

Additionally, the MAX9727 features extensive click-and-

pop suppression that eliminates any audible transient

sources internal to the device.

In some applications, the output of the device driving

the MAX9727 may have a DC bias. At startup, the

input-coupling capacitor is charged to the input

device’s DC-bias voltage through the input and feed-

back resistors of the MAX9727, resulting in a DC shift

across the capacitor and an audible click/pop. Delay

input device, to eliminate clicks-and-pops caused by

the input filter.

Applications Information

Amplifier Configurations

The MAX9727 works in many standard op-amp config-

urations such as inverting, noninverting, voltage follow-

er, summing, difference, active filters, and many others.

No special design considerations are required. The

DirectDrive architecture of the MAX9727 simplifies

many circuits due to the ground-referenced outputs.

Differential Input Configuration

Figure 2 shows a single channel of the MAX9727 con-

figured as a differential input amplifier. A differential

input offers improved noise immunity over a single-

ended input. In systems that include high-speed digital

circuitry, high-frequency noise can couple into the

amplifier’s input traces. The signals appear at the

amplifier’s inputs as common-mode noise. A differential

input amplifier amplifies the difference of the two

inputs, and signals common to both inputs are sub-

tracted out. When configured for differential inputs, the

voltage gain of the MAX9727 is set by:

The common-mode rejection ratio (CMRR) is limited by

the external resistor-matching. Ideally, to achieve the

highest possible CMRR the following condition should

be met:

R

R

R

R

F

IN

F

IN

1

1

2

2

=

A

R

R

V

F

IN

=

1

1

_______________________________________________________________________________________

9

IN_-

IN_+

OUT_

OUT_ =

(IN_ + - IN_-)

MAX9727

Figure 2. Differential Input Configuration