Differential Microphone Preamplifier with

Internal Bias and Complete Shutdown

_______________________________________________________________________________________

9

where:

Input Capacitors

The two differential microphone inputs and the single-

ended auxiliary input of the MAX4063 have on-chip

bias components, allowing the user to AC-couple any

signals into the input. The input resistance is 100k

Ω

(typ), so the capacitor size may be chosen accordingly

to define the LF rolloff desired. This can be calculated

as:

This assumes a low source impedance is driving the

inputs.

A further consideration for the differential input is the

effect of these series input capacitors on low-frequency,

common-mode rejection. Any mismatch in the values of

these two capacitors degrades the CMRR at frequen-

cies where the impedance of the capacitor is significant

compared to the input resistance of the amplifier—this is

usually most noticeable at low frequencies. One way to

avoid the need for matched or tight tolerance capaci-

tors is to deliberately oversize the values on the differ-

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.

AV

V

V

AUX

OUT

OUT

AUX_IN

=

− V

Figure 1. MAX4063 with Auxiliary Input Configuration

ADJ

7

AUX_IN

12

5

IN-

IN+

10

9

6

2.4V TO 5.5V

MAX4063

2

INT/AUX

8

OUT

OUT 4

GND

11

SHDN

SHDN

3

G2

G1

BIAS

1

14

100k

Ω