Comprehensive analysis of digital microphone technology and its development status

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 opening up a new situation for the application of electronic products such as microphones. ECM microphone manufacturers have been working hard to improve product sensitivity, signal-to-noise ratio and reflow soldering performance. Microphone analog-to-digital conversion chips, especially for micro-electromechanical systems (MEMS) microphones, are being introduced. Overall improvement of the above microphone performance.

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 transistors (JFETs) that have been widely used in the past decades have gradually been eliminated, and this market has emerged. The shift in the process of adding digital output to the microphone will be an important development in amplifier technology today, which is suitable for mobile phones, notebook computers and other portable microphone applications.

MEMS microphones are micro-microphones made by etching micro-electromechanical technology on a semiconductor with a pressure sensing diaphragm. As MEMS products become cheaper and more numerous, and the size, scalability and sound of silicon microphones The quality and other aspects also greatly exceed the traditional microphone. At the same time, MEMS also has features that simplify design in applications such as noise cancellation and beamforming. It is expected that global MEMS microphones will maintain an average annual growth rate of over 25%, and up to 1.1 billion annual shipments by 2013. scale. This also means that the digital microphone conversion chip market will have a scale of more than 100 million US dollars per year.

Based on this, Fairchild is emerging as a leader in the analog technology industry with high-performance ECM microphone digital conversion chips and is strategically entering the field of complete MEMS digital microphone solutions and noise cancellation systems.

Because the industry is so important and full of hope, in order to help readers deepen their technical understanding, improve product application capabilities and enhance their impression of Fairchild products, the author makes a basic introduction based on our microphone product parameters.

The basic structure of the digital microphone is to form the sound pressure to voltage conversion part based on the electret diaphragm or MEMS foundation, then integrate the very low noise voltage signal operational amplifier, high performance Σ·Δ analog to digital converter and pulse based A digital interface for density modulated output and supports stereo or time division multiplexing.

Of course, more importantly, 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 94dBSPL is –26dBFS, the signal-to-noise ratio (SNR) is 60-62dBc(A).

The PGA+ADC integrated noise floor is 6.3μVRMS, and the pure PGA noise floor is 3.2μVRMS.

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

The maximum input signal is designed to be 710mVP-P without affecting total harmonic distortion (THD).

Microphone gains of 12, 14, 16 dB are available for acoustic-to-electrical conversion sensitivity of -42 to -38 dBV/Pa.

Chip operating current ≤ 450μA.

The following authors explain the above parameters in the microphone product. In the acoustic equipment application, we introduce the sound pressure level (SPL: soundpressurelevel), which is the relative pressure parameter to characterize the sound. The sound pressure Lp is 20 micropascals (μPascal). A benchmark for characterizing the strong and small logarithmic results of sound pressure.

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

That is, Lp (1 Pascal) = 20 log 10 (1 Pa / 20 μPa) = 93.97 dB (SPL) ≈ 94 dB (SPL).

Then, we know that the sensitivity of a typical ECM microphone is -42 to -38dBV/Pa, which means that usually the microphone pickup front end receives a sound pressure of 1 Pascal and produces an average voltage fluctuation of -42 to -38dBV output to the amplification front end. dBV is the voltage input input of the microphone conversion based on 1Vrms (effective voltage or voltage rms). Therefore, -42dBV = 7.9mVRMS = 22.4mVP-P, thus, 120dBSPL

The sound pressure is absorbed by the front end of the microphone and will generate 120dBSPL–94dBPa/SPL–42dBV/Pa=-16dBV or 158.5mVRMS. At the same time, we also mention dBFS in the microphone index. The so-called dBFS is the ADC input voltage relative to the ADC reference voltage. Logarithmic representation: 20 & TImes; log10 (VIN & TImes; AV / VREF) = dBFS, we call the Fractional Full Scale (FracTIonalFullScale), usually we put the microphone front end sound pressure into 120dBSPL corresponding to the setting Av (amplifier gain) and VREF Let VIN & TImes; AV / VREF = 1, of course, these Av and VREF are generally configured inside the chip, so for this given set of chips, 0dBFS corresponds to 120dBSPL, -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 is the carrier. Therefore, we can use dBc on the logarithmic strength of the signal based on noise. When the product input sound pressure level 94dBSPL is -26dBFS, the signal-to-noise ratio is 62dBc(A), that is, when the signal-to-noise ratio is 88dBc(A) when 120dBSPL is 0dBFS, and the signal-to-noise ratio is (32dBBSPL). The SNR) is 0dBc, ie the effective signal is exactly equal to the noise intensity, so the product system noise (NoiseFloor) = 32dB (SPL), corresponding noise voltage = 32dBSPL - 94dBPA / SPL - 42dBV / Pa = -104dBV = 6.3μVRMS. It is also known that the input dynamic range of the digital microphone chip is 32-120 dB (SPL).

In general, by understanding this article, readers can understand that in order to meet the wide-ranging needs of users for a better listening experience on mobile devices, the new high-performance digital microphone is helping the target application to greatly improve the sound quality. It even offers more features such as noise determination and filtering, such as integrating multiple digital microphones for noise suppression and directional pickup. With the increasing use of portable devices in noisy environments and the urgent need to improve the quality of cell phone calls and multi-party calls, digital microphones will be fully prevalent.

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