# Sound Activity Monitor Circuit for Low Power Consumption of Always-On Microphone Applications
## 3 Simulation Verification and Results  
Figure 14 shows the layout of the proposed circuit and the regions of the main blocks of SF and SAM. The layout was performed based on the 0.18 \(\upmu\)m CMOS process, and the size of the circuit core was 0.71 mm \(\times\) 0.39 mm. Figure 15 shows the frequency response and noise characteristics of analog blocks. In Figure 15a, the dotted line and the solid line correspond to the frequency response characteristics when SF is in the low power mode and the normal power mode, respectively. In the low-power mode, a current of 100 nA flows through the input MOS of the source follower and has a bandwidth of about 56 kHz. In the normal power mode, a current of 7.6 \(\upmu\)A flows through the input MOS of the source follower and has a bandwidth of about 3.5 MHz. Figure 14: The layout of the proposed circuit and the regions of the main blocks of SF and SAM. Figure 15b shows the frequency response characteristics of the OA block. The high-pass cutoff frequency can be adjusted by changing the resistance \(R_{aa}\) value of the high-pass filter (HPF) of the OA block input part. The maximum amplification gain is 45.5 dB (172 V/V), and the low cutoff frequency can be adjusted in eight steps within the range of 2.9 Hz to 540 Hz. The OA block is a single-stage amplifier that directly utilizes an open-loop gain of 170 V/V for signal amplification. There are no stability issues as it does not use closed-loop feedback to get the correct gain, and there is no saturation in the absence of an input. Figure 15c shows frequency response characteristics of blocks continuously connected in the order of SF, OA, and AMP2. It can be seen that the low cutoff frequency varies from 30 Hz to 720 Hz according to the AMP2 feedback resistance setting value. The high cutoff frequency is also affected by the AMP2 feedback resistance value Rf setting and ranges from 1.4 kHz to 3.7 kHz. The amplification gain has a maximum of 63 dB (1395 V/V) and a minimum of 55 dB (560 V/V). Because 'OA + AMP2' blocks have much lower bandwidth than the SF bandwidth of 56 kHz in low-power mode, their combined frequency response characteristics show the same characteristics regardless of the power mode of SF. Figure 15d shows the noise characteristics of the combined blocks of SF, OA and AMP2. The SF noise characteristics vary depending on the power mode, and for this reason the noise characteristics of the 'SF + OA + AMP2' combined block vary depending on the power mode. The dotted line shows the noise characteristics when SF is in low power mode, and the solid line shows the noise characteristics when SF is in normal power mode. Comparing the noise levels of the 'SF + OA + AMP2' block based on the frequency of 1 kHz, the noise floor value has 72 nVrms/VHz in low power mode and 40 nVrms/VHz  
Figure 15: Characteristics of analog blocks: (**a**) Frequency response characteristics of SF; (**b**) Frequency response characteristics of OA; (**c**) Frequency response characteristics of SF + OA + AMP2; (**d**) Noise characteristics of SF + OA + AMP2. in normal power mode. The integrated noise for the range from 600 Hz to 2 kHz has a 2.6 \(\mu\)V\({}_{\text{rms}}\) in low power mode and a 1.7 \(\mu\)V\({}_{\text{rms}}\) in normal power mode. Due to the SAM connection architecture in this paper, the sensitivity of the MEMS transducer itself is the same whether or not SAM is added. Therefore, the overall microphone noise level for each power mode is determined by the noise level of the ROIC. Compared to the noise level of 1.7 \(\mu\)V\({}_{\text{rms}}\) in normal power mode, the noise level in low power mode is 2.6 \(\mu\)V\({}_{\text{rms}}\), so the noise level is about 50% higher. Numerically, therefore, the overall noise performance of the microphone is 50% higher in low power mode than in normal power mode. However, the meaning of noise level in normal power mode and low power mode can be slightly different. In normal power mode, low noise means whether small changes in volume can be measured when there is sound.