CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 implements a professional-grade studio or live performance microphone using up to 32 analog MEMS microphones connected to op amps and a difference amplifier. The circuit is designed to be very low-noise and its output is linear for acoustic inputs up to 131 dB SPL (sound pressure level). The ±9 V and +1.8 V power rails are generated from two voltage regulators powered from a single +9 V battery.
The ADMP411 consists of a MEMS microphone element and an impedance-matching amplifier. This microphone supports acoustic inputs up to 131 dB SPL and has a low-frequency response that is flat to 28 Hz. These features make this microphone ideal for full-bandwidth, wide dynamic range audio capture applications, such as in a recording studio or on stage.
The ADA4075-2 op amps are used to perform several different functions in this circuit, including a summing amplifier and all-pass filter. This op amp is low-noise, low-power, and low-distortion, making it a good choice for a battery-powered high-performance audio application.
The AD8273 converts the single-ended microphone signal into a differential signal that can be output on a standard microphone XLR connector. The gain setting resistors are internal to the difference amplifier, so it can create a high-performance differential signal with no external components. The difference amplifier has very low distortion, low noise and good output drive capability, making it a good choice for driving a differential microphone output.
The power supplies for this circuit are generated from an ADP1613 dc-to-dc switching converter and an ADP1720 linear regulator (see Figure 8 for schematic). The ADP1613 in a SEPIC- Ćuk configuration generates the ±9 V rails for the amplifiers and the ADP1720 generates the MEMS microphones' 1.8 V supply. These regulators efficiently generate the necessary voltage supplies with very low ripple.
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