Datasheet MAX9727 (Maxim) - 9

Quad Audio Line Driver with 3VRMS Output MAX9727 Shutdown Mode Differential Input Configuration
The MAX9727 features a low-power shutdown mode Figure 2 shows a single channel of the MAX9727 con- that reduces quiescent current consumption to less figured as a differential input amplifier. A differential than 0.1µA and extends battery life for portable appli- input offers improved noise immunity over a single- cations. Drive SHDN low to disable the amplifiers and ended input. In systems that include high-speed digital the charge pump. In shutdown mode, each amplifier’s circuitry, high-frequency noise can couple into the output resistance is high impedance to small signals. amplifier’s input traces. The signals appear at the The resulting output resistance seen by the load is amplifier’s inputs as common-mode noise. A differential determined by the series combination of the amplifier’s input amplifier amplifies the difference of the two external gain-setting resistors in parallel with the ampli- inputs, and signals common to both inputs are sub- fier’s shutdown output resistance. tracted out. When configured for differential inputs, the voltage gain of the MAX9727 is set by:
Click-and-Pop Suppression
In conventional single-supply audio amplifiers, the out- R A F = 1 put-coupling capacitor contributes significantly to audi- V RIN1 ble clicks and pops. Upon startup, the amplifier charges the coupling capacitor to its bias voltage, typically half where A the supply. Likewise, on shutdown, the capacitor is dis- V is the desired voltage gain in V/V. RIN1 must be equal to RIN2, and RF1 must be equal to RF2. charged. This results in a DC shift across the capacitor, which appears as an audible transient at the speaker The common-mode rejection ratio (CMRR) is limited by connected to the output of the power amplifier of the the external resistor-matching. Ideally, to achieve the audio system. Since the MAX9727 does not require out- highest possible CMRR the following condition should put-coupling capacitors, this problem does not arise. be met: Additionally, the MAX9727 features extensive click-and- pop suppression that eliminates any audible transient R R F1 F2 = sources internal to the device. R R IN1 IN2 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 RF1 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 the rise of SHDN 4 to 5 time constants based on RIN and CIN (4 x RIN x CIN), relative to the startup of the MAX9727 input device, to eliminate clicks-and-pops caused by RIN1 IN_- the input filter. OUT_
Applications Information
RIN2 IN_+
Amplifier Configurations
The MAX9727 works in many standard op-amp config- urations such as inverting, noninverting, voltage follow- RF2 er, summing, difference, active filters, and many others. RF1 OUT_ = (IN_ + - IN_-) No special design considerations are required. The RIN1 DirectDrive architecture of the MAX9727 simplifies RIN1 = RIN2, RF1 = RF2 many circuits due to the ground-referenced outputs. Figure 2. Differential Input Configuration
_______________________________________________________________________________________ 9