AN1534 - APPLICATION NOTE

3/4

Figure 1: ECM Microphone Pre-amp. with TS971

This bridge can also be supplied by an ASIC’s

"Vref out" pin or by another operator of the op-amp

connected as a buffer (using the TS972, a dual

op-amp, this can be implemented easily).

An important note on the impedance of the ampli-

put impedance of the amplifier stage. It must not

be too small to avoid the collapsing of the micro-

phone signal!

universal, however, you could omit it when attack-

ing an A/D converter. In this case, you only have

put voltage range of the converter. Thanks to its

Rail-to-Rail output, the TS971 simplifies the pro-

cess.

We’re coming now to the filter definition, when

looking at figure 1, we can see three filters: two

high pass and a low pass. Each has a 6dB/octave

attenuation factor.

Lower Cutoff Frequency (here approximately 79

Hertz) is the following

cutoff frequency is better set at a lower value than

tave) of low frequencies (here 59Hz).

Then for the low pass filter (optional), to calculate

imately 10.7kHz), we have the following formula::

The next step is to configure G or the gain of the

amplifier (here 90 or +39dB):

TS461/971

R8

82k

R7

820R

R6

100k

R5

100k

R1

1k

R2

2.2k

C7

3.3u

C1

10u

C6*

1u

C5

100n

C2*

10u

Vcc = 2.7V min.

Vcc

C8*

180p

Electret

Condenser

Microphone

Output

C4

2.2u

C3

4.7u

R3

15k

R4

18k

R

100R

C

47p

Optionnal

EMI filter

EMI

EMI

* Optional : refer to text

()

)

Hz

(

C

R

R

2

1

F

5

4

2

1

CL

×

+

×

π

×

≈

)

Hz

(

C

R

2

1

F

7

7

2

CL

×

×

π

×

≈

)

Hz

(

C

R

2

1

F

8

8

CH

×

×

π

×

=

microphone

the

of

Gain

)

R

R

R

(

)

R

R

1

(

G

2

4

4

7

8

×

+

×

+

≈