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AN1534 TS971 Based Electret Condenser Microphone Amplifier

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AN1534 TS971 Based Electret Condenser Microphone Amplifier AN1534 APPLICATION NOTE TS971 based Electret Condenser Microphone amplifier by R.CITTADINI & F.POULIN This application note explains how to implement Table 1: Acoustic Units Reference Table the TS971 as a microphone pre-amplifier for an Sound Electret Condenser Microphone (ECM). This type Acoustic Acoustic Pressure Activity Intensity Pressure of microphone has one of the best price to perfor- Level 2 mance ratio on the market. I in W/m p in Pa SPL in dB 1.E+02 200 140 Microphone pre-amplifiers are very common in to- Ear Damage 130 day’s appliances, digital appliances have adopted and kept the typologies used in analog ones. This 1.E+00 20 120 block is helping to interface the microphone to the 110 A/D converter by buffering, filtering and amplifying Night Club, Factory Floor 1.E-02 2 100 the microphone signal. 90 1 - DEVICE PRESENTATION 1.E-04 0.2 80 Many low noise amplifiers exist in the market. Old- 70 Conversation ies (but still goodies!) are mainly dual ones like 1.E-06 0.02 60 LM833, MC33078. But few are able to reach low voltages and are not available in today’s smallest 50 packages. 1.E-08 0.002 40 The TS97x is a family including single, dual and 30 quad low-noise operational amplifier. It features Recording Studio 1.E-10 0.0002 20 excellent audio characteristics: low distortion (0.003% THD @ F=1kHz) and a 4nV/sqrt(Hz) 10 equivalent input noise voltage with a 1/f corner @ IREF= 1.E-12 pREF= 0.00002 0 Minimum Level of Audition, Reference Level 100Hz. Thanks to those characteristics, it helps I SPL = 10 ×Log ( ) (dB ) keeping an optimal Signal to Noise ratio, a critical IREF point at the entry of the audio amplification chain! p SPL = 20 ×Log ( ) (dB) p These devices also allows a higher fidelity thanks REF to a 4V/µs Slew Rate and 12 MHz Gain Bandwidth Product. This enables the amplifier to cope with 2 - MICROPHONE CONSIDERATIONS quick variations of the input signal well over the audio bandwidth. Preliminary knowledge of Acoustic Intensity (in Watt/m2), Acoustic Pressure (in Pascal or Pa) and The family is available in compact packages like Sound Pressure Level or SPL (in Decibels or dB) SOT23-5 for TS971 or even the thin and rather is important. You can report to table 1 for more in- compact package like TSSOP for TS972/4. This formation. allows them to be used in portable and miniature ECM microphones follow more or less the same digital appliances like PDA or Cellular Phones and characteristics, however Gain and surrounding also in thin notebook computers. components may vary from one model to another. June 2002 1/4 AN1534 - APPLICATION NOTE We selected a popular model from Panasonic: the 3 - COMPONENTS CALCULATION WM-60A series. It’s an omni-directional micro- phone with the following main characteristics: Let’s look now on how to implement such an am- ❑ Operating: from 2 to 10V. plifier with TS971. You can refer to schematic on ❑ Sensitivity: -44dB +/- 5dB (0dB=1V/Pa). Figure 1 hereafter. We’ve chosen a non-inverter ❑ Impedance: less than 2.2kΩ typology to exploit to the best the low noise char- ❑ S/N ratio: more than 58dB acteristics of the device. Indeed, with an inverter ❑ Current Consumption: 0.5mA max configuration, the input resistor adds significant ❑ Ω Recommended Load Resistor: RL: 2.2k noise to the application. The sensitivity of the microphone defines its gain as per the following formula: First, let’s look on the behavior in DC mode. The first goal is to polarize the Electret Condenser Mi- Sensitivit y ( ) crophone. By using R and R , we can polarize it G = 10 20 (V / Pa) 1 2 mike around Vcc/2 as per below formula:. So with a -44dB sensitivity, we can conclude that ≈ Vcc the gain of the microphone is 0.0063V/Pa or Ipol − mike ( A ) 2 × (R + R ) 6.3mV/Pa. With this value, we can get a good idea 1 2 of the output voltage of the microphone. It would be around 12.6µV for a quiet room (2mPa or The only criteria is that this current must remain 40dB) and would reach approximately 6.3mV for below 0.5mA over the supply range (otherwise, the climax of a symphonic orchestra (1Pa or you can increase R1 value). 110dB). This sound data is with a source at 1 meter from the microphone. This reference is R1 is also acting together with C1 as a filter for the mandatory, the distance between the microphone power supply line of the microphone. Then in AC and the audio signal is illustrated by the Acoustic mode, C is fixing the gain of the microphone by Intensity (in Watt/m2). 1 allowing only R2 to act (and not R2+R1 as C1 is Let’s take the example of a conversation. It’s equivalent to a short circuit to the ground). And R2 equivalent to roughly 20mPa or 60dB SPL at 1 Ω meter. So an Acoustic Intensity of 1µW and 126µV must equal RL=2.2k for the microphone we’ve at the output of the microphone. The intensity de- chosen. In AC mode, this type of microphone can creases with the squared value of the distance be- be simplified and compared to a current source in tween the source and the microphone. So for a parallel with R2, hence a voltage source. distance of 5cm, you would get a value of 400µW. As per the Table 1 formulas, we can calculate the Then to avoid extra offset drift due to bias current "equivalent SPL value at 1m": 86dB, then we get mismatching, following resistor values need to the Acoustic Pressure: 0.4Pa which gives us the comply with the following rule: output of the microphone: R × R 0.4 x 0.0063 = 2.52mV (distance divided by 20 R ≈ R + R + 5 6 (Ohms ) 8 4 3 + and output voltage increased by the same ratio). R 5 R 6 We can summarize these considerations into the following checklist: The second step is, still in DC mode, to polarize ❑ What type of signal do you want to amplify? the reference pin of the TS971. It’s the inverting ❑ How powerful?At what distance? pin here that will be set at Vcc/2 by the R5 and R6 ❑ What are the minimum and maximum of bridge. C4 adds here additionnal filtering of this each above parameters? reference voltage. This configuration allows the bi- With these values, you will be able the calculate asing or the "centring" of the signal at mid-supply the microphone’s output voltage range and be voltage. Hence it allows to maximize the swing able to choose the right gain of the amplifier here- within the supply voltage range. This bias voltage after. just needs to be kept within V range. This Also, if you want to implement a noise canceling ICM function, you can also choose another type of mi- means VICM or Common Mode Input Voltage must crophone called bi-directional microphone or be at least 1.15V inside the supply voltage rails, noise canceling microphone. i.e. from Vdd+1.15 to Vcc-1.15V. 2/4 AN1534 - APPLICATION NOTE Figure 1: ECM Microphone Pre-amp. with TS971 Vcc = 2.7V min. + R1 C2* 1k 10u C1 + Optionnal R2 EMI filter 10u 2.2k C5 TS461/971 C6* REMI + Output 100n R4 18k 1u 100R CEMI 47p R8 Vcc 82k C8* Electret R5 180p Condenser 100k R7 Microphone R3 820R * Optional : refer to text + + 15k + C3 R6 C4 C7 4.7u 100k 2.2u 3.3u This bridge can also be supplied by an ASIC’s Lower Cutoff Frequency (here approximately 79 "Vref out" pin or by another operator of the op-amp Hertz) is the following connected as a buffer (using the TS972, a dual 1 op-amp, this can be implemented easily). F ≈ (Hz ) CL 1 × π × ()+ × 2 R 2 R 4 C 5 An important note on the impedance of the ampli- fier is that, in AC mode, R4 is equivalent to the in- Another high pass filter is made by C7 and R7. The put impedance of the amplifier stage. It must not cutoff frequency is better set at a lower value than be too small to avoid the collapsing of the micro- FCL to have a stronger reduction (i.e. -12dB/oc- phone signal! tave) of low frequencies (here 59Hz). 1 The coupling capacitor (C6) makes this application F ≈ (Hz ) CL 2 × π × × universal, however, you could omit it when attack- 2 R 7 C 7 ing an A/D converter. In this case, you only have Then for the low pass filter (optional), to calculate to adapt the bridge set by R5 & R6 to match the in- put voltage range of the converter. Thanks to its FCH or the Higher Cutoff Frequency (here approx- imately 10.7kHz), we have the following formula:: Rail-to-Rail output, the TS971 simplifies the pro- cess. 1 F = ( Hz ) CH × π × × We’re coming now to the filter definition, when 2 R 8 C 8 looking at figure 1, we can see three filters: two The next step is to configure G or the gain of the high pass and a low pass. Each has a 6dB/octave amplifier (here 90 or +39dB): attenuation factor. R R G≈ (1+ 8 )×( 4 )×Gainof themicrophone The high pass filter is built by C5, R4 and also R2. + R7 R4 R2 The theoretical formula to calculate FCL or the 3/4 AN1534 - APPLICATION NOTE This represents the gain of the amplifier in the double sided PCB with one side acting as ground "non-filtered" bandwidth area.
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