AN1534 - APPLICATION NOTE

2/4

We selected a popular model from Panasonic: the

WM-60A series. It’s an omni-directional micro-

phone with the following main characteristics:

Ω

The sensitivity of the microphone defines its gain

as per the following formula:

So with a -44dB sensitivity, we can conclude that

the gain of the microphone is 0.0063V/Pa or

6.3mV/Pa. With this value, we can get a good idea

of the output voltage of the microphone. It would

be around 12.6

µV for a quiet room (2mPa or

40dB) and would reach approximately 6.3mV for

the climax of a symphonic orchestra (1Pa or

110dB). This sound data is with a source at 1

meter from the microphone. This reference is

mandatory, the distance between the microphone

and the audio signal is illustrated by the Acoustic

Let’s take the example of a conversation. It’s

equivalent to roughly 20mPa or 60dB SPL at 1

meter. So an Acoustic Intensity of 1

µW and 126µV

at the output of the microphone. The intensity de-

creases with the squared value of the distance be-

tween the source and the microphone. So for a

distance of 5cm, you would get a value of 400

µW.

As per the Table 1 formulas, we can calculate the

"equivalent SPL value at 1m": 86dB, then we get

the Acoustic Pressure: 0.4Pa which gives us the

output of the microphone:

0.4 x 0.0063 = 2.52mV (distance divided by 20

and output voltage increased by the same ratio).

We can summarize these considerations into the

following checklist:

each above parameters?

With these values, you will be able the calculate

the microphone’s output voltage range and be

able to choose the right gain of the amplifier here-

after.

Also, if you want to implement a noise canceling

function, you can also choose another type of mi-

crophone called bi-directional microphone or

noise canceling microphone.

3 - COMPONENTS CALCULATION

Let’s look now on how to implement such an am-

plifier with TS971. You can refer to schematic on

Figure 1 hereafter. We’ve chosen a non-inverter

typology to exploit to the best the low noise char-

acteristics of the device. Indeed, with an inverter

configuration, the input resistor adds significant

noise to the application.

First, let’s look on the behavior in DC mode. The

first goal is to polarize the Electret Condenser Mi-

around Vcc/2 as per below formula:.

The only criteria is that this current must remain

below 0.5mA over the supply range (otherwise,

you can increase R1 value).

power supply line of the microphone. Then in AC

chosen. In AC mode, this type of microphone can

be simplified and compared to a current source in

Then to avoid extra offset drift due to bias current

mismatching, following resistor values need to

comply with the following rule:

The second step is, still in DC mode, to polarize

the reference pin of the TS971. It’s the inverting

pin here that will be set at Vcc/2 by the R5 and R6

bridge. C4 adds here additionnal filtering of this

reference voltage. This configuration allows the bi-

asing or the "centring" of the signal at mid-supply

voltage. Hence it allows to maximize the swing

within the supply voltage range. This bias voltage

be at least 1.15V inside the supply voltage rails,

i.e. from Vdd+1.15 to Vcc-1.15V.

)

Pa

/

V

(

10

G

)

20

y

Sensitivit

(

)

A

(

)

R

R

(

2

Vcc

I

2

1

mike

pol

+

×

≈

−

)

Ohms

(

R

R

R

R

R

R

R

6

5

6

5

3

4

8

+

×

+

+

≈