In recent years, with the development of microphone technology and small-signal analog-to-digital conversion technology, the electret condenser microphone (ECM) can increase the digital audio output, thus creating a new situation for the application of electronic products such as microphones. All along, ECM microphone manufacturers have been working to improve the sensitivity, signal-to-noise ratio and reflow soldering performance of products. The microphone analog-to-digital conversion chip, especially the conversion chip used in microelectromechanical system (MEMS) microphones, is being introduced Overall improve the above microphone performance.
With the addition of Fairchild and other well-known semiconductor companies and the introduction of ECM and MEMS microphone analog-to-digital conversion chips, the junction field effect transistor (JFET) that has been widely used in the past few decades has gradually been eliminated, and this market structure has emerged. The above change, adding digital output function to the microphone will be an important development of amplifier technology. This new technology is suitable for mobile phones, notebook computers and other portable microphone applications.
MEMS microphones are miniature microphones made by etching pressure sensing diaphragms on semiconductors through micro-electromechanical technology. As MEMS products become cheaper and more and more, and the size, scalability and sound of silicon crystal microphones Quality and other aspects also greatly exceed traditional microphones. At the same time, in applications such as noise cancellation and beamforming, MEMS also has the characteristics of simplifying the design. It is expected that the global MEMS microphones will maintain an average annual growth rate of more than 25%, and the annual shipments will reach 1.1 billion units by 2013. scale. This also means that the digital microphone conversion chip market will have an annual scale of more than 100 million US dollars.
Based on this, Fairchild Semiconductor, as a leader in the analog technology industry, is introducing high-performance ECM microphone digital conversion chips and is strategically entering the field of complete MEMS digital microphone solutions and noise cancellation systems.
Because this industry is so important and full of hope, in order to help readers deepen technical understanding, improve product application capabilities and strengthen their impression of Fairchild semiconductor products, the author gives you a basic introduction based on our microphone product parameters.
The basic structure of the digital microphone is based on the electret diaphragm or MEMS to form the sound pressure to voltage conversion part, and then integrates a very low noise voltage signal operational amplifier, high-performance Σ · Δ analog to digital converter and pulse-based Digital interface for density modulation output, and supports stereo or time division multiplexing.
Of course, what is more important is that digital microphone products need to meet the demanding performance indicators. Fairchild Semiconductor is committed to providing high-performance analog products to the industry and is providing the following excellent product indicators:
When the input sound pressure level is 94dBSPL or –26dBFS, the signal-to-noise ratio (SNR) is 60-62dBc (A).
The integrated noise floor of PGA + ADC is 6.3μVRMS, and the pure noise floor of PGA is 3.2μVRMS.
When the input sound pressure level is 94dBSPL or –26dBFS, the total harmonic distortion (THD) is "0.04%.
Without affecting the total harmonic distortion (THD), the design maximum input signal is: 710mVP-P.
Microphone gains of 12, 14, and 16 dB are available for microphones with a sensitivity of -42 to -38 dBV / Pa.
Chip working current≤450μA.
The following authors explain some of the above parameters in microphone products. In the application of acoustic equipment, we introduced the relative pressure parameter of sound pressure level (SPL: soundpressurelevel) to characterize the size of sound. The sound pressure Lp is 20 micropascals (μPascal) Use it as a benchmark to characterize the logarithmic result of the sound pressure.
Therefore, the sound pressure level corresponding to an effective sound pressure of 1 Pascal size is approximately 94 dBSPL.
That is, Lp (1Pascal) = 20log10 (1Pa / 20μPa) = 93.97dB (SPL) ≈94dB (SPL).
Then, we know that the sensitivity of a typical ECM microphone is -42 to -38dBV / Pa, which means that when the microphone front-end receives 1 Pascal sound pressure, it will produce an average voltage fluctuation of -42 to -38dBV and output it to the amplification front end. dBV refers to 1Vrms (effective voltage or root mean square voltage) as a reference to characterize the voltage input of the microphone conversion. Therefore, -42dBV = 7.9mVRMS = 22.4mVP-P, therefore, 120dBSPL
The sound pressure absorbed by the front end of the microphone will produce a voltage of 120dBSPL–94dBPa / SPL–42dBV / Pa = -16dBV or 158.5mVRMS. At the same time, we also mentioned dBFS in the microphone specifications. The so-called dBFS is the ADC input voltage relative to the ADC reference voltage. The logarithmic representation is: 20 & TImes; log10 (VIN & TImes; AV / VREF) = dBFS, we call it Fractional Full Scale (FracTIonalFullScale), usually when we input the sound pressure of the front end of the microphone as 120dBSPL, correspondingly set Av (amplifier gain) and VREF Make VIN & TImes; AV / VREF = 1. Of course, these Av and VREF are generally already configured inside the chip. Therefore, for the chip with this given setting, 0dBFS corresponds to 120dBSPL, and -26dBFS corresponds to 94dBSPL. Finally, let's talk about the calculation of the signal-to-noise ratio and noise. The signal-to-noise ratio is usually expressed in dBc or dB, c stands for carrier, so we generally use dBc when characterizing the logarithmic strength of the signal based on noise. The article mentions that when the input sound pressure level of the product is 94dBSPL or -26dBFS, the signal-to-noise ratio is 62dBc (A), which means that when 120dBSPL is 0dBFS, the signal-to-noise ratio is 88dBc (A), and when 32dBSPL is -88dBFS, the signal-to-noise ratio ( SNR) is 0dBc, that is, the effective signal and the noise intensity are completely equal, so the product system noise floor (NoiseFloor) = 32dB (SPL), the corresponding noise voltage = 32dBSPL-94dBPA / SPL-42dBV / Pa = -104dBV = 6.3μVRMS. At the same time, the input dynamic range of this digital microphone chip is 32-120dB (SPL).
In general, through the understanding of this article, readers can understand that in order to meet the wide demand of users for a better listening experience on mobile devices, the new high-performance digital microphones are helping target applications to greatly improve the sound quality. It even provides more functions such as noise determination and filtering, such as integrating multiple digital microphones to achieve noise suppression and directional sound pickup. With the increasing use of portable devices in a noisy environment and the urgent need to improve the sound quality of mobile phone calls and multi-party calls, digital microphones will definitely prevail.
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