Merge branch 'docs/add_adc_mic_hardware_design' into 'master'

docs(audio): add adc mic hardware design reference

See merge request ae_group/esp-iot-solution!1371

Author: leeebo

docs/_static/audio/adc_mic_hardware_ref_design.png

@@ -15,6 +15,19 @@ Features
 Reference Circuit
 ------------------
 
+The external reference circuit is shown below. You can choose between transimpedance amplifier or inverting/non-inverting amplifier circuits to amplify the MIC input signal. In most cases, using a transimpedance amplifier can achieve better signal-to-noise ratio. In this example, R29 is used to add DC bias for the MIC, using the general-purpose op-amp LMV321 as the operational amplifier, with R26 and C19 forming a feedback circuit to make the op-amp work in transimpedance amplification mode, where the resistance and capacitance are adjusted according to the MIC sensitivity. R31 and R30 are used to set the midpoint voltage to avoid crossover distortion, and R27 and R32 are used to provide DC bias to avoid distortion during ADC sampling. For more design reference information about front-end MIC amplifiers, please refer to `Single-Supply, Electret Microphone Pre-Amplifier Reference Design <https://www.ti.com/lit/ug/tidu765/tidu765.pdf>`__.
+
+.. note::
+    - Bias: ADC midpoint bias is necessary (such as Vref/2) to avoid ADC sampling distortion. By default, ESP32-C3's Vref is approximately 0.9V.
+    - Power supply: It is recommended to use LDO for MIC power supply regulation to avoid power supply fluctuations affecting ADC sampling. If external LDO is not used, please add separate RC filtering on the power supply side (in this example, R28 and C23 are used for filtering). It is recommended that MIC and op-amp have independent power supplies.
+    - It is recommended to select MIC with sensitivity greater than -46dB to avoid low signal-to-noise ratio after amplification. Adjust the amplification factor according to the microphone sensitivity.
+
+.. figure:: ../../_static/audio/adc_mic_hardware_ref_design.png
+    :align: center
+    :alt: ADC mic hardware reference design
+
+    ADC mic hardware reference design
+
 External Configuration for ADC Continuous
 ------------------------------------------
 

docs/en/audio/adc_mic.rst

@@ -2,7 +2,7 @@ ADC 麦克风
 ==============
 :link_to_translation:`en:[English]`
 
-使用 ADC 采集模拟麦克风数据，无需使用外部的音频 codec 芯片，适用于对采样率要求不太高且成本敏感的应用。在代码结构上采用 `esp_codec_dev <https://components.espressif.com/components/espressif/esp_codec_dev>`__ 的工程模式。
+使用 ADC 采集模拟麦克风数据，无需使用外部的音频 codec 芯片，适用于对采样精度要求不太高且成本敏感的应用。在代码结构上采用 `esp_codec_dev <https://components.espressif.com/components/espressif/esp_codec_dev>`__ 的工程模式。
 
 特性
 ------
@@ -15,6 +15,18 @@ ADC 麦克风
 参考电路
 ----------
 
+外部参考电路如下，可以选择跨阻放大器或者同相/反相放大电路对 MIC 输入信号进行放大。在大多数情况下，使用跨阻放大器可以取得更佳的信噪比，在本示例中 R29 用于为 MIC 添加直流偏置，使用通用运放 LMV321 作为运算放大器，通过 R26 与 C19 构成反馈电路，使运放工作在跨阻放大模式，其中阻容根据 MIC 的灵敏度进行调整。R31 与 R30 用于设定中位电压，以避免交越失真，R27 与 R32 用于提供直流偏置，避免 ADC 采集时失真。关于前端 MIC 放大器的更多设计参考信息，请参考 `Single-Supply, Electret Microphone Pre-Amplifier Reference Design <https://www.ti.com/lit/ug/tidu765/tidu765.pdf>`__。
+
+.. note::
+   - 偏置：ADC 端的中点偏置是必须的（如 Vref/2），避免 ADC 采集失真，默认情况下 ESP32-C3 的 Vref 约为 0.9V。 
+   - 电源：MIC 供电最好使用 LDO 进行稳压，避免电源波动影响 ADC 采样。如果不使用外部 LDO，请在电源侧进行单独的 RC 滤波（在本示例中 R28 与 C23 用于滤波）， MIC 推荐与运放独立供电。
+   - 推荐选择灵敏度大于 -46dB 的 MIC，避免放大后的信噪比过低。并根据 MIC 的灵敏度调整放大倍数。
+
+.. figure:: ../../_static/audio/adc_mic_hardware_ref_design.png
+    :align: center
+    :alt: ADC mic hardware reference design
+
+    ADC mic hardware reference design
 
 外部配置 ADC Continuous
 --------------------------