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This application note explains how to implement

the TS971 as a microphone pre-amplifier for an

Electret Condenser Microphone (ECM). This type

of microphone has one of the best price to perfor-

mance ratio on the market.

Microphone pre-amplifiers are very common in to-

day’s appliances, digital appliances have adopted

and kept the typologies used in analog ones. This

block is helping to interface the microphone to the

A/D converter by buffering, filtering and amplifying

the microphone signal.

1 - DEVICE PRESENTATION

Many low noise amplifiers exist in the market. Old-

ies (but still goodies!) are mainly dual ones like

LM833, MC33078. But few are able to reach low

voltages and are not available in today’s smallest

packages.

The TS97x is a family including single, dual and

quad low-noise operational amplifier. It features

excellent audio characteristics: low distortion

(0.003% THD @ F=1kHz) and a 4nV/sqrt(Hz)

equivalent input noise voltage with a 1/f corner @

100Hz. Thanks to those characteristics, it helps

keeping an optimal Signal to Noise ratio, a critical

point at the entry of the audio amplification chain!

These devices also allows a higher fidelity thanks

to a 4V/

µs Slew Rate and 12 MHz Gain Bandwidth

Product. This enables the amplifier to cope with

quick variations of the input signal well over the

audio bandwidth.

The family is available in compact packages like

SOT23-5 for TS971 or even the thin and rather

compact package like TSSOP for TS972/4. This

allows them to be used in portable and miniature

digital appliances like PDA or Cellular Phones and

also in thin notebook computers.

Table 1: Acoustic Units Reference Table

2 - MICROPHONE CONSIDERATIONS

Preliminary knowledge of Acoustic Intensity (in

Sound Pressure Level or SPL (in Decibels or dB)

is important. You can report to table 1 for more in-

formation.

ECM microphones follow more or less the same

characteristics, however Gain and surrounding

components may vary from one model to another.

Acoustic

Intensity

Acoustic

Pressure

Sound

Pressure

Level

Activity

I in W/m

2

p in Pa

SPL in dB

1.E+02

200

140

Ear Damage

130

1.E+00

20

120

110

Night Club, Factory Floor

1.E-02

2

100

90

1.E-04

0.2

80

70

Conversation

1.E-06

0.02

60

50

1.E-08

0.002

40

30

Recording Studio

1.E-10

0.0002

20

10

1.E-12

0

Minimum Level of Audition, Reference Level

)

dB

(

)

I

I

(

Log

10

SPL

REF

×

=

)

dB

(

)

p

p

(

Log

20

SPL

REF

×

=

AN1534

APPLICATION NOTE

TS971 based Electret Condenser Microphone amplifier

June 2002

by R.CITTADINI & F.POULIN