The ADAU1701 is a complete, single-chip audio system that integrates a 28/56-bit audio DSP, ADC, DAC, and a microcontroller-like control interface. It supports various signal processing functions such as balance, mixing, bass boost, multi-band dynamic processing, delay compensation, and stereo image expansion. These features help overcome real-world limitations in speakers, amplifiers, and listening environments, while enhancing the perceived audio quality in real-time.
The first circuit we introduce below uses an analog audio input, Class D output configuration with the ADAU1701 SigmaDSP codec, the low-power SSM2306 Class D amplifier, and the ADP3336 LDO regulator. This design offers a compact and efficient solution for audio applications requiring both line-level and amplified outputs.
The circuit shown in Figure 1 connects the ADAU1701 codec, which includes an integrated SigmaDSP® core, to the SSM2306 2W stereo Class D amplifier and the ADP3336 low dropout regulator. The ADAU1701 has two ADCs and four DACs built-in, allowing it to process one stereo audio signal and simultaneously output separately processed signals to both a line-level output and an amplified output. This enables different signal processing modes for each output, such as custom EQ, chip-level compressor settings, or spatialization effects tailored to specific speaker configurations.
The ADP3336 provides a 3.3 V power supply for the ADAU1701, while the SSM2306 operates directly from a 5 V system supply. The SSM2306 is known for its ultra-low idle current and high efficiency, making it ideal for low-power applications. It requires minimal external components and no bulky inductors, resulting in a small system footprint.
The DAC output of the ADAU1701 is connected to the SSM2306 through a resistor and capacitor at each amplifier input. A 0.10 μF capacitor in series with a 13.0 kΩ resistor creates a 28 Hz high-pass filter between the ADAU1701 output and the SSM2306 input. These resistors also set the amplifier gain to approximately 6 dB. The full-scale output of the ADAU1701 is 0.9 V rms, which the SSM2306 amplifies to 1.8 V rms (5.09 V pp) when operating at 5 V. This matches the clamp level of the amplifier precisely.
The circuit uses the multipurpose (MP) pin of the ADAU1701 to control the active-low shutdown pin of the SSM2306. Combined with a 10 kΩ pull-up resistor, this setup allows the SigmaDSP program to disable the Class D amplifier smoothly without any noise or popping.
To stabilize the SSM2306's Class D amplifier output, a ferrite bead and a 1.0 nF capacitor are placed on each speaker pin. This ensures stable operation even under varying load conditions.
The SSM2306’s power supply is taken directly from a 5 V source, while the ADAU1701 needs a 3.3 V regulated supply provided by the ADP3336. The output voltage of the ADP3336 is set to 3.3 V using a 140 kΩ and 78.7 kΩ feedback resistor network. A 1.0 μF capacitor between the output and ground helps maintain a stable regulator output, and another 1.0 μF capacitor at the input decouples stray inductance between the board and the 5 V supply.
In addition to the ADAU1701, this circuit can be adapted to use other SigmaDSP processors with integrated DACs, such as the ADAU1761. The ADAU1702 can also be used as a replacement, differing mainly in the size of the SigmaDSP program and data memory. Other Class D amplifiers like the SSM2301, SSM2302, and SSM2304 offer similar performance but with slight differences in functionality. For example, the SSM2301 is a mono amplifier rather than a stereo one.
This digital audio system solution based on the ADAU1701 and ARM7 offers a powerful and flexible platform for audio applications, combining advanced signal processing with efficient power management. The accompanying diagrams illustrate various circuit configurations and component layouts for better understanding and implementation.
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