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Application note
Pre-amplifying the analog output of a MEMS microphone
Pierre Sennequier
Introduction
MEMS microphones are an innovative way of recording sound.
STMicroelectronics provides a good portfolio of MEMS microphones. Products include the
MPxAxx family (e.g. MP23AB02B).
Such products have an analog output. Generally, this output signal is quite low and needs to
be amplified to reach the desired level required for the next stage (from microphone level to
line level).
This application note describes how to perform this “pre-amplification”.
January 2015
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Contents
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Contents
1
Output signal amplitude and gain setting . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4
Op amp choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5
Designing the circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6
Inverting configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7
Differential output configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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Output signal amplitude and gain setting
The analog signal on the output of the microphone contains the audio signal plus some
noise.
The amplitude of the audio output signal depends on the acoustic pressure.
The relationship between sensitivity (So) and gain (Gmicro) at 1 kHz is given in the
MP23AB02B datasheet and is calculated using Equation 1.
Equation 1
G micro = 10
S
-----o20
Where So is measured in dBV/Pa and Gmicro in V/Pa.
In terms of scale, 20 µPa is the minimum level of hearing, while talking produces an acoustic
pressure around 20 mPa. Acoustic pressure can also be expressed in dB SPL (sound
pressure level) where: dB SPL ↔ 20 log (P/20 µPa). So, 1 Pa is equivalent to 94 dB SPL.
The acoustic and electrical relationships of the MP23AB02B are illustrated in Figure 1.
Figure 1. Acoustic and electrical relationships of the MP23AB02B
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Output signal amplitude and gain setting
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Note that acoustic pressure drops with distance from the source.
Assuming free field conditions, if we double our distance from the source, the acoustic
pressure is divided by:
•
2 if the source is considered to be a point like a speaker (i.e. -6 dB SPL)
•
√2, if the source is considered to be a line such as a road (i.e. -3 dB SPL)
Knowing the acoustic pressure applied to the microphone and the desired amplitude after
the amplification stage tells us how much the gain should be.
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Noise
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The maximum gain setting (Gmax) is calculated using Equation 2 by considering the
maximum sensitivity of the microphone (Somax), the maximum output signal amplitude
needed after the amplification stage (Voutrms_max), and the maximum acoustic pressure
(Pmax).
Equation 2
Vout rms_max
G max = -------------------------------S
10
omax
-------------20
P max
If Voutrms = 1 Vrms, Somax = -35 dBV/Pa, and Pmax = 1 Pa, Gmax = 56.
2
Noise
Overall noise includes the noise generated by the microphone and the noise generated by
the amplification stage.
The noise level (erms) on the output of the microphone is calculated using Equation 3 by
considering the signal-to-noise ratio (SNR) as given in the MP23AB02B datasheet.
Equation 3
e rms = 10
S o – SNR
-------------------------20
Where erms is expressed in Vrms and SNR in dBA.
erms is shown as the noise floor in Figure 1.
Note that the SNR is A-weighted in order to consider noise as a human ear would hear it.
SNR is the difference (in dB) between a 94 dB SPL signal (whose amplitude is So (dBV/Pa)
* 1 (Pa) = So (dBV)) and the noise level (dBV).
The typical noise level in the MP23AB02B is 7 µVrms.
This is equivalent to a white noise source (eo), over a 20 kHz bandwidth, as calculated using
Equation 4.
Equation 4
e rms
e o = -------------------20000
Where eo is expressed in nV/√Hz.
In case of the MP23AB02B, eo is 56 nV/√Hz.
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Distortion
Independent noise sources (measured in Vrms) can be combined together by taking the
square root of the sum of their squares.
If we consider a positive gain amplification of the output of the microphone (as shown in
Figure 2) and if the noise level generated by the op amp is only half the noise level
generated by the microphone (erms), the resulting overall noise is only 1.12 * erms.
Then, considering the same 20 kHz bandwidth, using an op amp whose equivalent white
noise density is only 28 nV/VHz only increases the noise level by 12 % for the MP23AB02B.
To avoid decreasing significantly the noise level and therefore the signal-over-noise ratio, an
op amp with a white noise of less than 20 nV/√Hz is recommended and less than 10 nV/√Hz
is perfect.
Note that the overall noise level at the amplification output depends greatly on the shape of
the gain stage (cutoff frequencies and filter orders).
Also, it is important when designing a circuit to bear in mind that resistors are a source of
noise and that a resistor, R kΩ, generates a white noise source of 4√(R) nV/√Hz.
So, it is important to keep impedances relatively low.
3
Distortion
The signal distortion on the output of the microphone depends on the acoustic pressure as
specified in the datasheet.
For the MP23AB02B, the signal distortion is less than 0.5 % at 94 dBSPL (which is
equivalent to 1 Pa).
For high acoustic pressure, the distortion level increases. A 10 % distortion level is shown in
Figure 1. The microphone can be used from the noise floor up to the distortion level. This is
its dynamic range.
The signal distortion should be compared with the total harmonic distortion (THD) which is
introduced by the op am. Generally op amps are not limiting for this parameter.
But we should bear in mind that the THD increases when frequency increases, for large
output signals and low load values.
A pure sine waveform on the output of the op amp can calculated using Equation 5.
Equation 5
V P sin ( 2πf )
The maximum slope of such a waveform is 2πfVp.
Therefore, the op amp needs to have a slew rate greater than 2πfVp.
If we consider a 20 kHz waveform with a 3 Vpp amplitude, the op amp minimum slew rate
must be 0.19 V/us. A margin (of at least a factor of two) should be considered on this value
to ensure the smallest distortion of the signal.
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Op amp choice
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Op amp choice
The MP23AB02B microphone supply voltage is between 1.6 V and 3.6 V.
If we want to have only one power supply domain in our application, we should use a low
voltage op amp which is output rail-to-rail (especially at these low voltages where losing 1 V
really makes a difference).
Considering the noise level, slew rate, and THD requirements, the recommended
operational amplifiers are given in Table 1.
Table 1. Recommended op amps
Vcc
(V)
Noise
(nV/√Hz)
Slew rate
(V/µs)
TS971
2.7 to 10
4
4
TS982
2.5 to 5.5
17
0.7
TS922
2.7 to 12
9
1.3
MC33078
5 to 30
4.5
7
Rail-to-rail
output
Minimum
load (Ω)
2k
Yes
16
32
No
600
Note that all these op amps are available in automotive grade versions.
The first choice is the TS971. For driving low loads the TS922 or TS982 should be used. If
the op amp has a dedicated high-voltage power supply (single or dual supply), the
MC33078 should be used.
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5
Designing the circuit
Designing the circuit
A typical pre-amplification circuit is shown in Figure 2.
Figure 2. MEMS microphone amplification circuit
R4 and R5 are used to generate a voltage reference to bias the input common mode voltage
of the op amp at Vcc/2.
C5 allows the noise of the power supply (and of the equivalent R4 and R5 resistors) to be
reduced. Generally, this noise would be amplified by the op amp.
R6 and C6 create a low pass filter which also helps to reduce noise, resulting in an overall
second order, low-pass filtering.
R1 and C1 allow AC coupling of the microphone signal. The op amp is DC biased by Vcc/2
and in AC i.e. above ~1/2π R1C1 (16 Hz) its input receives the microphone signal.
Note that if the op amp is used with a dual supply configuration, ground is used as the input
common mode voltage (R1 is directly connected to ground).
R2 and C2 create a high-pass gain so as not to amplify the DC biasing of the op amp
(including input offset voltage). The cutoff frequency is fhp = 1/2π R2C2 which is 59 Hz.
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Designing the circuit
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R3 and C3 create a low-pass gain so as not to amplify noise beyond the audio bandwidth.
The cutoff frequency is flp = 1/2π R3C3 which is 10.5 kHz.
Between fhp and flp the voltage gain of the amplification stage is calculated using
Equation 6.
Equation 6
R3
G = 1 + -------R2
Figure 3 shows the transfer function of the proposed schematics.
For high frequencies the gain is 1.
In order to remove the input noise beyond the audio frequency, an additional low-pass RC
filter can be used on the OUT node.
Figure 3. Voltage gain vs frequency
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C8 and C9 decoupling capacitors should be placed as close as possible to the op amp and
microphone.
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Inverting configuration
Inverting configuration
Other circuits are of course possible. For example, Figure 4 shows an inverting
configuration where the main architecture (without any filtering and biasing of the input at
Vcc/2) has a gain of –R2/R1.
Figure 4. Inverting configuration
To design a circuit with a differential output, we can simply cascade an inverting stage (with
R2 = R1) to the output of the first stage amplifier.
Both outputs would then be with the same amplitude, but with a phase inversion.
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Differential output configuration
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Differential output configuration
In case a differential output signal is needed, we recommend the use of the TS472 device,
as illustrated in Figure 5.
Figure 5. Differential output configuration
The TS472 comes with standby mode which is effective on the biasing of the microphone if
the 2-V bias embedded in the chip is used.
The footprint on the PCB is reduced because the TS472 is available in flip-chip and
because this device requires less surrounding passive components than with a standard op
amp.
Finally, if the microphone is far from the op amp, the differential input of the TS472 allows a
better immunity to ground noise.
The gain for this device is determined by the Rgs value or it can be set to 20 dB if the gain
select pin is connected to the bias pin (or a voltage above 1 Vdc).
Note:
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C2 and C4 decoupling capacitors should be placed respectively as close as possible to the
op amp and microphone.
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Conclusion
Conclusion
This application notes describes the key parameters related to pre-amplification of the
output signal of an analog MEMS microphone (MP23AB02B). A solution is proposed based
on the TS971 op amp. For a differential output configuration, we can consider using the
TS472. The information in this document should allow you to design your own circuit
suitable for your application. You can find the best-adapted MEMS microphone and op amp
from STMicroelectronics.
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Revision history
Table 2. Document revision history
Date
Revision
21-Jan-2015
1
Changes
Initial release
DocID026988 Rev 1
11/12
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AN4598
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