Differential Microphone Preamplifier with

Internal Bias and Complete Shutdown

10

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Input voltages should be sufficiently small such that the

output is not clipped in either differential or common-

mode application. The topology used in the MAX4063

means that the CMRR actually improves at higher dif-

ferential gains—another advantage of using differential

sensing.

Auxiliary Input

The auxiliary input is a single-ended input intended to

be used with a jack-socket type microphone input

(Figure 1). Internal DC-bias components (as on the

main inputs) allow the input signal to be AC-coupled.

Mechanically switched jack sockets can be used in

conjunction with the INT/AUX select pin, allowing the

auxiliary microphone input to be automatically selected

when a jack socket is inserted.

Microphone Bias Voltage

On the MAX4063 thin QFN package, connect the

exposed paddle (backside of PRS) to the ground

plane. The MAX4063 has an integrated low-noise, low-

output impedance bias voltage. An optimum electret

bias resistor can be set externally. This gives a low-

noise, flexible solution that can run from 2.4V to 5.5V,

which is suitable for hand-held devices such as PDAs

that typically have audio power supplies in the 3V

region (Figure 2).

Output

DC Bias

In shutdown mode, the bias voltage is disabled. OUT

and OUT have a low-noise, DC-bias voltage indepen-

dent of the power supplies, resulting in superior PSRR

performance. The MAX4063 outputs are high imped-

ance when the part is in shutdown mode.

Differential Output

The MAX4063 features a differential output stage (OUT

and OUT), allowing optimum performance when con-

nected to ADCs and CODECs with differential inputs.

This differential output is particularly useful in designs

where the microphone preamplifier is mounted some

distance away from the CODEC/ADC, as the low-imped-

ance, differential line provides excellent noise rejection

and immunity (Figure 4).

Applications Information

Shutdown Mode

The MAX4063 features a low-power, complete shut-

down mode. When SHDN goes low, the supply current

drops to 0.3µA, the output enters a high-impedance

state, and the bias current to the microphone is

switched off. Driving SHDN high enables the amplifier.

SHDN should not be left floating.

Power Supplies and Layout

The MAX4063 operates from a 2.4V to 5.5V single sup-

ply. Bypass the power supply with a 0.1µF capacitor to

ground. In systems where analog and digital grounds

are available, the MAX4063 should be connected to the

analog ground.

IN-

IN+

BIAS

10

9

5

3

AUX_IN

MICROPHONE

INPUT

12

6

SHDN

SHDN

2.4V TO 5.5V

MAX4063

INT/AUX 2

OUT 8

OUT 4

ADJ

7

GND

11

1

14

G1

G2

Figure 2. MAX4063 Used for Biasing a Microphone