MEMS Microphone peak output signal level

Hi Jerad-

I have seen the following question about Analog Devices MEMs microphones from one of our customers.  Can you address this?

How can I determine the peak output voltage from an analog MEMS  microphone? Some data sheets only show the peak sound pressure level (SPL). I  need to know the voltage at that SPL to ensure that the microphone doesn't clip  my preamp circuit.

Thanks,

AndyR

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  • 0
    •  Analog Employees 
    on Jan 12, 2012 12:56 AM

    Hi Andy,

    An analog microphone's peak output voltage can be derived from the sensitivity and maximum SPL specs. Let's use the ADMP404, which has a -38 dBV sensitivity and 120 dB SPL maximum acoustic input, as an example.

    The -38 dBV sensitivity means that the ADMP404 will have a 12.5 mV rms output for a 94 dB SPL (1 Pascal) acoustic input. This is calculated using the equation 10^(-38 dBV/20) = rms voltage output. Note that both this voltage output and acoustic input are given as rms units. In peak levels, the microphone will output a 17.8 mV peak signal with a 97 dB SPL (peak) input. This voltage is equivalent to -35 dBV, which is naturally 3 dB higher than the sensitivity spec which is given for rms signals.

    The maximum (peak) acoustic input that the microphone can input before clipping is 120 dB SPL, which is 23 dB above the reference sensitivity level (97 dB SPL peak) that is described above. To get the peak voltage at this SPL you can just add 23 dB to the -35 dBV level to get a -12 dBV level at the maximum acoustic input. This can be converted to a peak voltage by peak output voltage = 10^(-12 dBV/20), or about 250 mV peak. Double this to get the peak-to-peak output voltage, which will be about 500 mV. Now you can use this signal level to design your system's gain staging so that you have sufficient headroom and can avoid clipping the microphone's output signal.

    You can use the equations above for different microphones with different sensitivities and maximum acoustic inputs to calculate their peak output voltages. Newer MEMS microphone data sheets, such as the one for the ADMP504, include the maximum output voltage in the specifications table.

    Although it would be good practice to make you work through the calculations yourself, I'll list the maximum output voltage for Analog Devices' MEMS microphones here:

    ADMP401: 158 mV peak

    ADMP404: 250 mV peak

    ADMP405: 250 mV peak

    ADMP504: 250 mV peak

Reply
  • 0
    •  Analog Employees 
    on Jan 12, 2012 12:56 AM

    Hi Andy,

    An analog microphone's peak output voltage can be derived from the sensitivity and maximum SPL specs. Let's use the ADMP404, which has a -38 dBV sensitivity and 120 dB SPL maximum acoustic input, as an example.

    The -38 dBV sensitivity means that the ADMP404 will have a 12.5 mV rms output for a 94 dB SPL (1 Pascal) acoustic input. This is calculated using the equation 10^(-38 dBV/20) = rms voltage output. Note that both this voltage output and acoustic input are given as rms units. In peak levels, the microphone will output a 17.8 mV peak signal with a 97 dB SPL (peak) input. This voltage is equivalent to -35 dBV, which is naturally 3 dB higher than the sensitivity spec which is given for rms signals.

    The maximum (peak) acoustic input that the microphone can input before clipping is 120 dB SPL, which is 23 dB above the reference sensitivity level (97 dB SPL peak) that is described above. To get the peak voltage at this SPL you can just add 23 dB to the -35 dBV level to get a -12 dBV level at the maximum acoustic input. This can be converted to a peak voltage by peak output voltage = 10^(-12 dBV/20), or about 250 mV peak. Double this to get the peak-to-peak output voltage, which will be about 500 mV. Now you can use this signal level to design your system's gain staging so that you have sufficient headroom and can avoid clipping the microphone's output signal.

    You can use the equations above for different microphones with different sensitivities and maximum acoustic inputs to calculate their peak output voltages. Newer MEMS microphone data sheets, such as the one for the ADMP504, include the maximum output voltage in the specifications table.

    Although it would be good practice to make you work through the calculations yourself, I'll list the maximum output voltage for Analog Devices' MEMS microphones here:

    ADMP401: 158 mV peak

    ADMP404: 250 mV peak

    ADMP405: 250 mV peak

    ADMP504: 250 mV peak

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