# Sound Activity Monitor Circuit for Low Power Consumption of Always-On Microphone Applications
###### Abstract  
A novel sound activity monitor (SAM) circuit for low power consumption of always-on microphone applications is presented. To reduce average power consumption, the ultra-low-power SAM is essential and operates a readout integrated circuit (ROIC) in low power mode with silent input or in normal power mode with voice input. A novel SAM with an architecture that does not include an envelope detector is proposed to achieve low power consumption. A new architecture is also proposed to improve MEMS sensitivity by connecting the SAM input to the source follower (SF) output instead of connecting the SAM input to the MEMS port already connected to the SF. In addition, in order to prevent inefficient frequent operation mode conversion, a feature of delaying the transition to the low-power mode after the sound is silenced is implemented. The proposed architecture is designed and verified based on the standard 0.18 \(\upmu\) CMOS process. The SAM, which consists of two-stage amplifiers (OA, AMP2), comparators, and a logic circuit, consumes a 1 \(\upmu\)A current. The analog path consisting of SF, OA, and AMP2 in low power mode has a maximum amplification gain of 63 dB and a noise of 72 \(\upmu\)V\({}_{\text{rms}}\)/\(\upmu\)Hz at 1 kHz. microphone; ROIC; low power; sound activity monitor (SAM) +
Footnote †: journal: Applied Sciences  
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Footnote 1: email: [EMAIL_ADDRESS]. As a traditional method of reducing power consumption, it can be approached from the viewpoint of reducing power consumption of major components. If the microphone ROIC is divided into two main parts, it is an analog signal processing part and an analog to digital converter (ADC). In order to reduce power consumption in the analog signal processing part, an approach of minimizing the current consumption of the sub-block circuit can be used. Alternatively, the average power consumption can be reduced by changing the amplifier's current consumption according to the required noise level [6; 7]. Moreover, obviously, reducing the power consumption of the ADC is also important in reducing the power consumption of the microphone [8; 9]. However, in terms of power consumption reduction, the high-level improvement approach may have a greater effect than the low-level improvement approach. A low-level approach means improving the sub-block circuit level, and a high-level approach means improving the top architecture or utilizing a use scenario. Therefore, in this paper, the use scenario is used as a strategy to reduce the power consumption of the microphone in the corresponding application. Figure 1 shows a microphone operation scenario to achieve low average power consumption. The ROIC consists of a low drop-out voltage regulator (LDO) for generating power, a drive bias generator (DR) for driving the MEMS transducer, a main channel for measuring sound, and a sound activity monitor (SAM) for detecting the presence of sound. Figure 1a shows no sound input and the ROIC operating in a low power mode. In low power mode, only elements for the MEMS transducer drive and SAM functions operate normally, while others operate in low-power or power-off mode. Therefore, the SAM must be operated at extremely low power consumption. When the SAM detects an external sound, as illustrated in Figure 1b, the ROIC operates in a normal power mode. In normal power mode, the SAM detects silence and putsthe ROIC in a low power mode again. Figure 1: Microphone operating scenarios for low average power consumption: (**a**) Low power mode operation when there is no external sound activity; (**b**) Normal power mode operation when there is external sound activity. Previous works on the SAM function of microphones are referenced in several papers [10; 11; 12; 13]. In 2010, [PERSON] showed an example of controlling the analog full channel as always on and only the digital part for speech analysis on/off [10].