Circuit function and advantages
The circuit shown in Figure 1 allows two iMEMS® analog microphones to interface with an audio codec. The ADMP404 consists of a MEMS microphone element and an output amplifier. Analog Devices' MEMS microphones have high signal-to-noise ratio (SNR) and flat broadband frequency response, making them an excellent choice for high-performance, low-power applications.
Up to two ADMP404 analog microphones can be input to the two ADCs of the low-power codec ADAU1761.
Figure 1. Connection of microphone and audio codec (schematic schematic, power decoupling and all connections are not shown)
Circuit description
The ADMP404 microphone is connected to the ADAU1761 LINN and RINN input pins. These pins are connected to the inverting input of the internal PGA. The only passive component required for this circuit is a capacitor: each ADMP404 requires a 0.1 μF bypass capacitor, and each microphone output requires a 2.2 μF capacitor in series. The decoupling capacitor should be placed as close as possible to the ADMP404 VDD pin (Pin 3). ADAU1761 LINP and RINP should be directly connected to the codec CM pin.
ADMP404 power is provided by the ADAU1761 MICBIAS pin. MICBIAS can be set to 0.9 × AVDD or 0.65 × AVDD, and the AVDD allowed by the ADAU1761 is 1.8 V to 3.3 V. The ADMP404 VDD power supply should be between 1.5 V and 3.6 V.
The sensitivity of ADMP404 is −38 dBV. In most applications, the microphone output must have additional gain, which is provided by the internal PGA of the ADAU1761. The maximum gain of the input PGA is 35.25 dB, the gain setting step is 0.75 dB, and the circuit after the PGA can also provide a fixed gain of 20 dB.
The PGA input and the AC coupling capacitor between the microphone and the codec form a high-pass filter. The −3 dB corner frequency of this filter is 1 / (2πRC), where C is the size of the capacitor and R is the input impedance of the codec for a given PGA gain setting. For a +20 dB gain setting (9.1 kΩ input impedance) and 2.2 μF capacitor, the corner frequency of this high-pass filter is 8 Hz. Increasing the PGA gain setting will increase the corner frequency of the filter, while increasing the capacitance will lower the corner frequency. Refer to the ADAU1761 data sheet for details on input impedance in different settings.
Register settings
The following register bit fields of the ADAU1761 must be set to enable the PGA and ADC to handle the left channel microphone input. If two microphones are used, repeat setting 1 to setting 4 for the register bit field that controls the right channel. These settings are as follows:
MX1EN = 0b1 in register R4: enable the left channel input mixer.
LDBOOST [1: 0] in register R5 = 0b01 (0 dB additional gain) or 0b10 (20 dB additional gain).
LDVOL [5: 0] of register R8: Set the PGA input gain within the range of −12 dB to 35.35 dB.
Register R8 LDEN = 0b1: enable the left differential input path.
Register R10 MBI = 0b0 (0.90 × AVDD) or 0b1 (0.65 × AVDD): Set the output level of MICBIAS.
MBIEN = 0b1 in register R10: enable MICBIAS output.
LDMUTE = 0b1 in register R8: the left differential input channel is unmuted. To avoid popping and clicking sounds, this bit field should be set last.
Figure 2 shows a screenshot of the SigmaStudio â„¢ analog input register control.
Figure 2. SigmaStudio PGA input configuration: ADMP404 microphone input to ADAU1761
At the default performance setting (MPERF = 0), the MICBIAS output provides up to approximately 2 mA to the microphone VDD power supply. The maximum power consumption of the ADMP404 is 250 μA, so there is no need to set this bit to high performance (MPERF = 1), even when powering two microphones. The high-performance bias mode of the codec is only needed when biasing the electret microphone, not for powering the MEMS microphone.
For the complete design support package for this circuit note, please refer to http: // CN0207-DesignSupport.
Common changes
ADAU1361 can also be used instead of ADAU1361 to set up this circuit. The main difference between the two is that the ADAU1761 contains a SigmaDSP® processor core, but the ADAU1361 does not. You can also use the ADAU1781 SigmaDSP codec.
Just remove an ADMP404 microphone and its associated capacitors to set up a mono microphone circuit that uses a single ADMP404. In the mono configuration, the other connections remain unchanged.
You can also use the analog microphone ADMP401 or ADMP405 instead of ADMP404. The sensitivity of the ADMP401 is −42 dBV, while the ADMP404 is −38 dBV. The low frequency cut-off frequency of ADMP405 is 200 Hz, and ADMP404 is 100 Hz, except that they are identical. The higher cut-off frequency allows the ADMP405 to reduce low-frequency wind noise.
Circuit evaluation and testing
You can use the evaluation boards for ADMP404 and ADAU1761 and easily connect them as described below.
Equipment requirements
SigmaStudio GUI software requirements for PC are as follows: Windows® 7, Windows Vista, Windows XP Professional or Home Edition (SP2); 128 MB RAM (256 MB recommended); 50 MB available hard disk space; 1024 × 768 screen resolution; USB 1.1 /2.0 data port.
In addition, the ADAU1761 evaluation board (EVAL-ADAU1761Z) and the ADMP404 evaluation board (EVAL-ADMP404Z-FLEX) are also required.
start using
EVAL-ADMP404Z-FLEX has three output lines: VDD, GND and OUTPUT. The VDD line should be connected to J15 or J18 on the EVAL-ADAU1761Z board to supply power from the ADAU1761 MICBIAS pin. The output wire of the ADMP404 board can be connected to the top of a mono 3.5 mm audio plug, and the ground wire to the ring. This plug is connected to the two analog input jacks of the ADAU1761 evaluation board: J20 and J22. The ADAU1761 evaluation board has a 10 μF AC coupling capacitor instead of the 2.2 μF capacitor shown in Figure 1.
Then, please refer to the EVAL-ADAU1761Z documentation to perform software installation, setup, and system operations.
Use SigmaStudio software to set and adjust the ADAU1761 registers and SigmaDSP core. To download SigmaStudio, please visit: http: //
Functional block diagram
The ADAU1761 evaluation board (EVAL-ADAU1761Z) document provides system setup instructions and a complete circuit board schematic. The only external connection required is a USB connection to the PC and the audio output of the ADAU1761 evaluation board.
Setup and test
For more detailed information on circuit description, jumper settings, configuration, and testing, please refer to the EVAL-ADAU1761Z evaluation board documentation.
Products used in this circuit:
Product description can provide sample product model
ADAU1761 SIGMADSP® stereo with integrated PLL, low power, 96 kHz, 24-bit audio codec ADAU1761BCPZ
ADMP404 omnidirectional microphone with bottom port and analog output ADMP404ACEZ-RL7
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