Differential Microphone Preamplifiers with

Internal Bias and Complete Shutdown

10

______________________________________________________________________________________

AUX_IN

9

4

IN-

IN+

7

6

5

2.4V TO 5.5V

MAX4062

2

GND

8

G2

G1

BIAS

100k

Ω

OUT 3

INT/AUX

10

1

x10

Figure 1. MAX4062 with Auxiliary Input Configuration

amplifier by sizing the output capacitor appropriately.

The input impedance matching on the differential input

is typically 1%, allowing input capacitor matching to be

effective at improving low-frequency PSRR.

Common-Mode Rejection Ratio

The common-mode rejection ratio (CMRR) refers to the

amount of rejection that the amplifier is capable of pro-

viding to any signal applied equally to the IN+ and IN-

inputs. In the case of amplifying low-level microphone

signals in noisy digital environments, it is a key figure of

merit. In audio circuits, this is generally measured for

mode gain.

Input voltages are sufficiently small such that the output

is not clipped in either differential or common-mode

application. The topology used in the MAX4061/

MAX4062 means that the CMRR actually improves at

higher differential 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

MAX4060

The MAX4060 has a microphone bias voltage designed

to comply with the Microsoft/Intel PC99/2001 audio

standard. It features source impedance of greater than

2k

Ω, and delivers more than 2V of bias when loaded

with a current of 800μA. This limits operation of this part

to supplies between 4.5V to 5.5V (see Figure 2).