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Comprehensive analysis of digital microphone technology and development status

Author:admin   AddTime:2021-04-09

In recent years, with the development of microphone technology and small-signal analog-to-digital conversion technology, electret condenser microphones (ECM) can increase digital audio output, thus creating a new situation for the application of electronic products such as microphones. For a long time, ECM microphone manufacturers have been committed to improving product sensitivity, signal-to-noise ratio and reflow soldering performance. Microphone analog-to-digital conversion chips, especially those used in micro-electromechanical system (MEMS) microphones, are being launched. Improve the performance of the above-mentioned microphones as a whole.

With the addition of a large number of well-known semiconductor companies such as Fairchild Semiconductor and the introduction of ECM and MEMS microphone analog-to-digital conversion chips, the junction field effect transistor (JFET) that has been commonly used in the past few decades has gradually been eliminated, and this market has also emerged. The above change, adding a digital output function to the microphone will be an important current 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 the number continues to increase, the size, scalability and sound of silicon crystal microphones The quality and other aspects also greatly exceed traditional microphones. At the same time, in terms of noise cancellation, beamforming and other applications, MEMS also has the characteristics of simplifying design. It is expected that the global MEMS microphone will maintain an average annual growth rate of more than 25%, and it will reach 1.1 billion annual shipments 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 launching 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 their technical knowledge, improve product application capabilities, and strengthen their impression of Fairchild Semiconductor products, the author will give you a basic introduction based on the parameters of our microphone products.

The basic structure of digital microphone is based on electret diaphragm or MEMS to form the sound pressure to voltage conversion part, and then internally 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, the more important thing is that digital microphone products need to meet 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 comprehensive noise floor of PGA+ADC is 6.3μVRMS, and the noise floor of pure PGA is 3.2μVRMS.

When the input sound pressure level is 94dBSPL, which is –26dBFS, the total harmonic distortion (THD) "0.04%.

Without affecting the total harmonic distortion (THD), the design maximum input signal is: 710mVP-P.

The microphone gain of -42 to -38dBV/Pa for acoustic-electric conversion sensitivity is 12, 14, 16dB optional.

Chip working current ≤450μA.

The following author elaborates on some of the above parameters in the microphone product. In the application of acoustic equipment, we introduce the relative parameter of sound pressure level (SPL: soundpressurelevel) to characterize the size of the sound. The sound pressure Lp is 20 micropascals (μPascal). As a benchmark to characterize the logarithmic result of the sound pressure.

Therefore, the sound pressure level corresponding to the effective sound pressure of 1 Pascal is about 94dBSPL.

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, that is to say, the average voltage fluctuation of -42 to -38dBV will be generated when the microphone pickup front end receives 1 Pascal sound pressure and output to the amplifier front end. dBV is the voltage input that is converted by the microphone using 1Vrms (effective voltage or voltage root mean square value) as a reference. Therefore, -42dBV=7.9mVRMS=22.4mVP-P, therefore, 120dBSPL

The sound pressure absorbed by the microphone front end 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 FullScale, usually we set the Av (amplifier gain) and VREF when the microphone front-end sound pressure input is 120dBSPL. Set VIN&TImes;AV/VREF=1. Of course, these Av and VREF are generally 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 signal-to-noise ratio and noise. The signal-to-noise ratio is usually expressed in dBc or dB, and c stands for carrier, so we can generally use dBc when characterizing the logarithmic strength of the signal based on noise. The article mentions that when the product input sound pressure level 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 noise intensity are exactly 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, it can be seen that 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 users' wide-ranging needs for a better listening experience on mobile devices, 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 high-noise environments and the urgent need to improve the sound quality of mobile phone calls and multi-party calls, digital microphones are bound to become fully popular.

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