南频-应用笔记-如何将MEMS的三线输出当两线使用

Introduction
MEMS microphones are being used to replace electret condenser microphones (ECMs) in audio circuits. These two types of microphones perform the same function, but the connection between the microphone and the rest of the system is different for ECMs and MEMS microphones. This application note will explain those differences and provide design details for a simple MEMS microphone based replacement circuit.

ECM Connections to Audio Circuits

An ECM has two signal leads – output and ground. The microphone is biased through a dc bias on the output pin. This bias is typically sourced through a 2.2 kΩ resistor and the signal between the microphone output and the preamp input is ac-coupled.麦克风是通过输出脚的一个直流偏置来偏置的 ，这个偏置通常是通过一个2.2K的电阻，输出和地，通过直流偏置，麦克风输出和放大器输入是交流耦合。

Figure 1.ECM circuit connection
A common use of an ECM is as an in-line voice microphone in a headset connected to a phone. In this application, the connector between the headset and the phone has four pins – left audio output, right audio output, the microphone signal, and ground. 左、右，麦克风，地。The ECM’s output signal and dc bias voltage are carried on the same signal line in this design. The dc bias voltage supplied to the ECM is typically about 2.2 V. 麦克风供电是2.2V

MEMS Microphone Differences

An analog MEMS microphone does not use an input bias voltage on its signal pin. 信号脚上面没有偏置Rather, it is a three-terminal device with separate pins for power, ground, and output. The Vdd pin is usually supplied with 1.8 to 3.3 V. The MEMS microphone’s signal output is biased at a dc voltage, usually close to 0.8 V. In a design, this output signal is typically ac-coupled. ?输出信号偏置是直流，大概0.8V

Circuit Changes to Replace an ECM with a MEMS Microphone

The basic challenge to a MEMS microphone in a system originally designed around a MEMS microphone is that there are not separate signals for power and the microphone output, 用MEMS替换ECMsuch as with the headset microphone. A MEMS microphone can be used in a design like this if some small changes are made to the circuit. First, the dc bias provided downstream in the signal chain must be isolated from the microphone’s output signal. 首先从麦克风输出信号中分离出直流偏置Second, this dc bias must be used to power the MEMS microphone without allowing the microphone’s output signal to interfere with the power supply. 直流偏置必须用来开启麦克风不让麦克风的输出信号和电源。The dc bias isolation can be provided with an ac-coupling capacitor and the MEMS microphone power can be provided from a carefully-designed circuit that serves as a voltage divider and a low pass filter. 直流偏置隔离可以用一个交流耦合电容，麦克风电源可以从设计仔细的电路作为一个电压触发器和低通。

Figure 2 . MEMS microphone with a single wire for power and output signal

Figure 2 shows an example of a design that achieves this. Capacitor C1 ac-couples the microphone output so that it doesn’t see the 2.2 V dc bias. C1交流耦合麦克风输出，这个就没有看到2.2V的直流偏置。 A capacitor of at least 1 μF is a good choice in this position.至少1uF
Resistor R1 forms a voltage divider with the MEMS microphone to bring the 2.2 V down to 1.8 V at the VDD pin. ?R1减压到1.8V。 The necessary resistor size can be calculated by modeling the microphone as a resistor through which a fixed current is flowing. 电阻大小可以通过仿真麦克风来计算出来，固定的通过的电流。The ADMP504’s typical supply current when VDD = 1.8 V is 180 μA. Using Ohm’s law with 1.8 V on VDD, this microphone can be modeled as a 10 kΩ resistor. The voltage divider equation used to solve for the appropriate value for resistor R1 is:
模型出来就是10K欧。电压除法器公司用来解决适当的值是R1:
[Microphone VDD] = [Bias voltage] × (10 kΩ/(10 kΩ + R1)). 电源=偏置电压。
From there, it can be calculated that a 2.2 kΩ resistor is needed at R1 to divide the voltage from the 2.2 V bias voltage to the 1.8 V microphone VDD. Two different models of this voltage divider are shown in Figure 3 . On the left, the ADMP504 microphone is modeled as a 180 μA current source, and on the right the microphone is modeled as a 10 kΩ resistor with a 1.8 V VDD.电压仿真和电流仿真。

Figure 3 .Voltage divider models
Capacitor C2 and the R1 form a high pass filter to filter the microphone’s audio output from its voltage supply signal. RC组成了高通滤波器来从他的电压供应信号中滤除麦克风的音频输出This filter corner frequency should be well below the lower filter corner of the microphone itself. 这个滤波拐点频率一定要比麦克风的低频低很多For the ADMP504, this corner is at 100 Hz. 504的拐点频率是100赫兹 A 4.7 μF capacitor and 2.2 kΩ resistor give a filter with a 15 Hz corner frequency. 这个电路就给了15hz的拐点频率。A larger capacitor will lower this corner frequency even further. 电容继续加大会更加降低这个频率。The equation for the low-pass filter corner is ?3 dB point = 1/(2π(R1)(C2)), where R1 is the resistor in the voltage divider and C2 is the low pass filter capacitor. ?
Figure 4 shows a complete circuit using the ADMP504 MEMS microphone and appropriate resistor and capacitor values.

Figure 4 .Circuit with ADMP504 MEMS Microphone

Conclusion
The circuit described here will allow a MEMS microphone to be used in a design where there are not separate signals available for power and the microphone output. The circuit replaces one resistor and one capacitor that is used in an ECM-based circuit with one resistor and two capacitors.
Revision History
2/13—Revision 0: Initial Version
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