ST AN1534 APPLICATION NOTE

1.E-06

0.02

REF

TS971 based Elec tret Condens e r Microphone am plifie r

This application note explains how to implem ent the TS971 as a microphone pre-amplifier for an Electret Condenser Microphone (ECM). This type of microphone has one of the bes t price to performance 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

AN1534

APPLICATION NOTE

by R.CITTADINI & F.POULIN

Table 1: Acoustic Units Reference Table

Acoustic Intensity

I in W/m

1.E+02 200 140

1.E+00 20 120

1.E-02 2 100

1.E-04 0.2 80

Acoustic Pressure

p in Pa SPL in dB

Sound

Pressure

Activity

Level

Ear Damage

130

110

Night Club, Factory Floor

Many low noise amplifiers exist in the market. Oldies (but still goodies!) are mainly dual ones like LM833, MC33078. But few are able to reach low voltages and are not avai lable in today’s sm allest packages.

The TS97x is a fami ly 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 v oltage 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 al so allows a higher fidelity thanks to a 4V/µs Slew Rate and 12 MHz Gain Bandwidth Product. This enab les the amplifier to cope with quick variations of the input signal well over the audio bandwidth.

The family i s available in com pact 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 portab le and miniature digital appliances like PDA or Cellular Phones and also in thin notebook computers.

Conversation

1.E-08 0.002 40

×=

(Log10SPL

(Log20SPL

Recording Studio

)dB()

REF

)dB()

REF

1.E-10 0.0002 20

= 1.E-12 p

REF

= 0.00002 0 Minimum Level of Audition, Reference Level

2 - MICROPHONE CONSIDERATIONS

Preliminary knowledge of Acoustic Intensity (in Watt/m

), Acoustic Pressure (in Pascal or Pa) and Sound Pressure Lev el or SPL (in Decibels or dB ) is important. You can report to table 1 for more in- formation.

ECM microphones follow m ore or less the same characteristics, however Gain and surrounding components may vary from one model to another.

June 2002

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AN1534 - APPLICATION NOTE

We selected a popular model from Panasonic: the

WM-60A series. It’s an omni-directional microphone with the following main characteristics:

❑ Operating: from 2 to 10V. ❑ Sensitivity: -44dB +/- 5dB (0dB=1V/Pa). ❑ Impedance: less than 2.2kΩ ❑ S/N ratio: more than 58dB ❑ Current Consumption: 0.5mA max ❑ Recommended Load Resistor: R

The sensitivity of the microph one defines its gain as per the following formula:

ySensitivit

(

mike

So with a -44dB s ensitivity, we can conclu de 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 12.6µV for a quiet room (2mPa or 40dB) and wou ld reach approximately 6.3m V 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 di stance b etwe en t he m icrophone and the audio signal is illustrated by the Acoustic Intensity (in Watt/m

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 1µW and 126µV at the output of the microphone. The intensity decreases with the squared value of the distance between the source and the microphone. So for a distance of 5cm, you would get a value of 400µW. As per the Table 1 formulas, we can calculate the "equivalent SPL value a t 1m": 86dB, then we get the Acoustic Press ure: 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 sum marize these consi derations into the following checklist:

)

: 2.2kΩ

)Pa/V(10G

❑ What type of signal do you want to amplify? ❑ How powerful?At what distance? ❑ What are the minimum and maximum of

each above parameters?

With these values, you will be able the cal culate the microphone’s output voltage range and be able to choose the right gain of the amplifier hereafter.

Also, if you want to implement a noise canceling function, you can also choose another type of microphone called bi-directional microphone or noise canceling microphone.

3 - COMPONENTS CALCULATION

Let’s look now on h ow to implemen t such an am plifier with TS971. You can refer to schem atic on Figure 1 hereafter. We’ve chosen a non-inverter typology to exploit to the b est the low noi se characteristics of the device . Indeed, with an inverter configuration, the input resistor adds significant noise to the application.

First, let’s look on the beha vior in DC mode. T he first goal is to polarize the El ectret Cond ens er M icrophone. By using R

and R2, we can polarize it

around Vcc/2 as per below formula:.

≈

mikepol

−

Vcc

+×

)A(

)RR(2

The only criteria is t hat this current must remain below 0.5mA over the supply range (otherwise, you can increase R1 value).

is also acting together with C1 as a filter for the

power supply line of the microphone. Then in AC mode, C allowing only R

is fixing the gain of the microphone by

to act (and not R2+R1 as C1 is

equivalent to a short circuit to the ground). And R must equal RL=2.2kΩ for the microphone we’ve chosen. In AC mode, this type of microphone can be simplified and comp ared to a current sou rce in parallel with R

, hence a voltage source.

Then to avoid ext ra of fset drift due to bias c urrent mismatching, following resistor values need to comply w it h the f o llow ing rule:

++≈

RRR

348

)Ohms(

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 biasing or the "centring" o f the signal at mid-supply voltage. Hence it allows to maximize the swing within the supply voltage range. This bias voltage just needs to be kept within V means V

or Common Mode Input Voltage must

ICM

range. This

ICM

be at least 1.15V inside the s upply voltage rails, i.e. from Vdd+1.15 to Vcc-1.15V.

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