AN1534 Datasheet, PDF (1/4 Pages) STMicroelectronics – TS971 based Electret Condenser Microphone amplifier

AN1534
APPLICATION NOTE
TS971 based Electret Condenser Microphone amplifier
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.
by R.CITTADINI & F.POULIN
Table 1: Acoustic Units Reference Table
Acoustic
Intensity
I in W/m²
1.E+02
Acoustic
Pressure
p in Pa
200
Sound
Pressure
Level
SPL in dB
140
130
Activity
Ear Damage
1.E+00
20
120
110
Night Club, Factory Floor
1.E-02
2
100
90
1.E-04
0.2
80
1.E-06
0.02
70
Conversation
60
50
1.E-08
0.002
40
1.E-10
0.0002
30
Recording Studio
20
10
I_REF= 1.E-12 p_REF= 0.00002
0 Minimum Level of Audition, Reference Level
SPL = 10 ×Log ( ^I ) (dB)
I_REF
SPL = 20 ×Log( ^p ) (dB)
p_REF
2 - MICROPHONE CONSIDERATIONS
Preliminary knowledge of Acoustic Intensity (in
Watt/m²), Acoustic Pressure (in Pascal or Pa) and
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.
June 2002
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