Circuit Design and Realization of MEMS Microphone (4)

Current consumption of digital microphones is generally higher than that of analog interface microphones with comparable performance levels. This difference is due to the analog-to-digital conversion being done in the microphone, not later in the signal chain. There are other factors that affect the power consumption of current digital microphone systems. Power consumption depends on supply voltage level, clock frequency and capacitive loading in the system. The higher the clock frequency, the faster the clock and data lines have to go back and forth from one state to another. The higher the capacitive load, the more current is drawn to drive these lines.

Device Microphone Signal Path Requirements – Bandwidth

The sampling rate (Fs) determines the bandwidth of the PCM system

Circuit Design and Realization of MEMS Microphone (4)

The following table lists common audio bandwidth and corresponding sample rate requirements. For an audio system to have 20kHz bandwidth, the sample rate must be 40kHz or higher. 48kHz and 44.1kHz (used in cd) are typical ratios. In communication systems, full-band audio is implemented through VoIP (Voice over Internet Protocol) and VoLTE (Voice over Long Term Evolution) technologies.

Circuit Design and Realization of MEMS Microphone (4)

Another common sample rate is 96kHz. It may be required in a microphone system to capture ultrasonic frequencies up to 48kHz. Using a 96kHz sample rate is unlikely to improve audio quality in the audible frequency range.

Even a 32kHz or 16kHz sample rate is high enough if the goal is not to cover the entire 20kHz audible sound bandwidth. For things like lower transmission bit rates, lower system current consumption, simpler systems, or lower prices, lower sampling rates can be considered. Even the 16kHz sample rate and corresponding 8kHz audio bandwidth enable “HD sound” quality and use the AMR-WB (Adaptive Multi-Rate Wideband; ITU-T/3GPP) codec for GSM phones.

Current consumption of digital microphones is generally higher than that of analog interface microphones with comparable performance levels. This difference is due to the analog-to-digital conversion being done in the microphone, not later in the signal chain. There are other factors that affect the power consumption of current digital microphone systems. Power consumption depends on supply voltage level, clock frequency and capacitive loading in the system. The higher the clock frequency, the faster the clock and data lines have to go back and forth from one state to another. The higher the capacitive load, the more current is drawn to drive these lines.

The current consumption of high performance digital microphones may be too high for some applications or use cases. There may also be other reasons for wanting to change the characteristics of the microphone. Multi-mode microphones address the need for microphone versatility. The most common alternative usage mode available in PDM MEMS microphones is low-power mode, which typically reduces the performance of the microphone to achieve lower current consumption.

In PDM interface microphones, this mode is usually controlled by changing the frequency of the microphone clock. Of course, this means that the device system (/codec) must have the required clock frequency available and a way to switch from one frequency to another. For example, in normal use mode, 2.4 or 3.072 MHz, 768kHz can be used in low power mode.

The system should also take into account that switching from one mode to another may not be completely trouble-free. To avoid any unwanted pops or clicks in the microphone system output, the microphone signal may have to be temporarily muted during mode switching.

Also, about EMC

Electromagnetic Compatibility (EMC), which describes the capabilities of a microphone.

・Operates in equipment free from electromagnetic environment interference
・Do not interfere with other systems in the device

EMC problems with microphones can manifest in different ways:

・The microphone is disturbed by radiated or conducted interference in the device
・Poorly designed digital microphones (e.g., the signal rises and falls too fast, gets damaged)

ground) can emit interference that can affect the antenna located very close to the microphone

・The microphone – actually a relatively large grounded metal box – passively interferes with the function

adjacent antenna

・This can be mitigated by moving the microphone away from the antenna or improving the grounding

There are many sources of noise in connected devices such as smartphones:

・Wireless connection antenna (cellular network, wi-fi, etc.) outputs both electric and magnetic fields
・ Interference with other signal lines connected to the microphone
・Indirect coupling: For example, radiated radio frequency interference generated in or within the device itself

The external source is coupled into the signal trace and from there to the microphone

・ Reason for noise
・ Electrically noisy components (such as RF power systems) may add noise to the microphone signal trace

RFI occurs when it couples into the microphone signal line or directly into the microphone itself. This interference propagates to the microphone’s output signal and creates an audible interference known as “TDMA noise.” GSM cellular devices use Time Division Multiple Access (TDMA) technology at 800 to 900 MHz and 1800 to 1900 MHz. Transmission pulses can be high at the audible 217Hz frequency and power level, causing the 217Hz pulses to couple into the microphone output signal.

Microphone implementations must be well implemented so that the microphone is well protected from all radiated and conducted interference present in wirelessly connected devices.

The microphone signal should be filtered, e.g. capacitors and inductors

・A capacitor (C) passes high frequencies, depending on its capacitance value, so it can be used to short excess high frequencies to the ground of the device
・ The Inductor (L) allows low frequencies to pass and blocks high frequencies, so it can be used in series on the signal line to filter radio frequency interference
・ Combinations of capacitors and inductors yield the best filtering results; for example, so-called pi filters (see image below)

Circuit Design and Realization of MEMS Microphone (4)
Figure 12 Pi Filter

The Links:   SX19V007-Z2A MSP430F5510IRGCR TIMMALCD

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