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Noise-cancelling headphone amplifiers suit mobile phone designs

austriamicrosystems has introduced two audio headphone amplifiers with active noise cancelling circuitry that are said to result in a 60% reduced bill of materials, compared to other noise cancelling architectures. The AS3501 suits feed-forward topologies, whereas the AS3502 is better suited for feedback. Notably, the AS3501 can be used in systems where the microphone is acoustically close to the loudspeaker.

The devices operate in stand-alone mode for accessories, or embedded mode, enabling integration into portable multimedia devices. These include mobile phones, as Oliver Jones, austriamicrosystems marketing manager for audio products intimates when he says that the AS3501/2 will allow end users to 'listen to conversations and music with improved clarity, without having to turn up the volume to unreasonable levels.'

The speakers can be configured in single ended mode (2x 34mW @1.5V, 16Ohm) or Bridge Tied Load (1x 109mW @1.5V, 16Ohm) and the line-in gain is adjustable to allow for locally controlled volume setting. With a less than 100dB SNR and less than 0.1% total harmonic distortion, austriamicrosystems says that the AS3501/2's sound quality is an important differentiator over existing active noise cancellation solutions, which can introduce artificial noise when being activated.

austriamicrosystems also claims that these parts are low power. The Class AB headphone amplifier in this device consumes approximately 5mW (or 3.4mA
@ 1.5V) to which the ANC circuitry adds roughly 3mW (or 2mA @ 1.5V). In addition, designers will need to include the microphone consumption in their calculations, which can be anywhere from 200uA to 1mA (x 2, left ear and right ear) depending on the type and quality of microphone. However, Jones notes: “I have often found when listening to music on an MP3 player that I have to turn the volume down to obtain an impression of equivalent music volume when the ANC is activated. This implies that the ANC can have a positive influence on power consumption.”

With most portable devices powered typically from an AAA battery, the AS3501/2 tolerates 1.0V to 1.8V supply, and contains the necessary charge pumps to enable true ground operation, as well as providing a supply voltage for the microphones.

As previously mentioned, a characteristic of portable multimedia devices is that they use a wide range of microphones and speakers. This part offers a user-definable register setting that facilitates different device configurations, allowing the signal processing to be tailored to different component combinations. Another feature is the monitor mode, which allows the user to attenuate Line-in, while amplifying the ambient noise. This is typically used to hear announcements in airports, without the need to remove the headset.

The AS3501 is available in a 24pin 4x4mm QFN package; whilst the AS3502 comes in a 32pin 5x5mm QFN package. Both are suitable for operating environments ranging from -20°C to +70°C. For product specific information, to download data sheets and other documentation, please visit www.austriamicrosystems.com/audio/ANC.

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