Input Amplifier

Differential Input

The MAX9701 features a differential input structure,

making it compatible with many CODECs and offers

improved noise immunity over a single-ended input

amplifier. In devices such as cellular phones, high-fre-

quency signals from the RF transmitter can be picked

up by the amplifier’s input traces. The signals appear at

the amplifier’s inputs as common-mode noise. A differ-

ential input amplifier amplifies the difference of the two

inputs, any signal common to both inputs is canceled.

Single-Ended Input

The MAX9701 can be configured as a single-ended

input amplifier by capacitively coupling either input to

GND, and driving the other input (Figure 6).

DC-Coupled Inputs

The input amplifier can accept DC-coupled inputs that

are biased within the amplifier’s common-mode range

(see the

Typical Operating Characteristics). DC cou-

pling eliminates the input-coupling capacitors, reduc-

ing component count to potentially two external

components (Figure 7). However, the highpass filtering

effect of the capacitors is lost, allowing low-frequency

signals to feed through to the load.

Component Selection

Input Filter

that removes the DC bias from an incoming signal. The

AC-coupling capacitor allows the amplifier to automati-

cally bias the signal to an optimum DC level. Assuming

zero-source impedance, the -3dB point of the highpass

filter is given by:

interest. Use capacitors whose dielectrics have low-volt-

age coefficients, such as tantalum or aluminum electrolyt-

ic. Capacitors with high-voltage coefficients, such as

ceramics, may result in increased distortion at low fre-

quencies.

Other considerations when designing the input filter

include the constraints of the overall system and the

actual frequency band of interest. Although high-fidelity

audio calls for a flat-gain response between 20Hz and

20kHz, portable voice-reproduction devices such as

cellular phones and two-way radios need only concen-

trate on the frequency range of the spoken human voice

(typically 300Hz to 3.5kHz). In addition, speakers used

f

RC

dB

IN IN

−

=

3

1

2

π

1.3W, Filterless, Stereo Class D Audio

Power Amplifier

14

______________________________________________________________________________________

MAX9701

0.1

μF

10

μF

2.5V TO 5.5V

0.47

μF

0.47

μF

0.47

μF

0.47

μF

SINGLE-ENDED

LEFT AUDIO INPUT

SINGLE-ENDED

RIGHT AUDIO INPUT

OUTL+

OUTL-

OUTR+

OUTR-

GAIN1

GND

PGND

SYNC

INL+

INR+

INL-

INR-

GAIN2

SHDN

Figure 6. Single-Ended Input

MAX9701

0.1

μF

10

μF

2.5V TO 5.5V

OUTL+

OUTL-

OUTR+

OUTR-

GAIN1

GND

PGND

SYNC

INL+

INR+

INL-

INR-

GAIN2

SHDN

CODEC BIASED TO 1/2 MAX9701 COMMON-MODE VOLTAGE.

CODEC

Figure 7. DC-Coupled Inputs