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Digital microphones have extended to mobile devices

2021-03-08 33
In recent years, microphones have been rapidly developed, enabling electret condenser microphones (ECM) to increase digital audio output, thus creating a new situation for microphones. Providing product sensitivity, signal-to-noise ratio and reflow soldering performance has always been the goal of ECM manufacturers, and it has also achieved considerable results in these years.

Digital microphones used junction field-effect transistors (JFET) more than ten years ago, and they have been eliminated. As a result, the structure of this market has changed greatly. Digital microphones have become the current trend. Adding digital output functions to microphones will It will be an important current development of amplifier technology, and this technology will be widely used.

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

The digital microphone conversion chip market will have an annual scale of more than 100 million U.S. 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 Density tone


Digital microphones have extended to mobile devices


The digital interface of the system 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) is less than 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-mentioned 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 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 sound pressure absorbed by the microphone front end will produce 120dBSPL–94dBPa/SPL–42dBV/Pa=-16dBV voltage or 158.5mVRMS. At the same time, we are in the microphone indicator It also mentions dBFS. The so-called dBFS is the logarithmic expression of the ADC input voltage relative to the ADC reference voltage, that is: 20×log10 (VIN×AV/VREF)=dBFS, we call it Fractional Full Scale, usually we take the microphone When the front-end sound pressure input is 120dBSPL, correspondingly set Av (amplifier gain) and VREF to make VIN×AV/VREF=1. Of course, these Av and VREF are generally configured inside the chip, so for the chip with the 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. It is also known that the input dynamic range of this digital microphone chip is 32-120dB (SPL).

This article tends to be technical, with many parameters. What you don’t understand can be directly understood as, in order to satisfy users to get a better hearing experience on mobile devices, then there are many manufacturers developing high-performance digital microphone sound quality. Digital microphones are getting better and better.


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