AKM公司的AK4688是一款立体声音频CODEC。内嵌支持异步操作的24位ADC和DAC输入/输出数据接口，通过外部电阻调节预放大器的输入范围来支持线路输入。内部集成一个电荷泵，通过3.3V单电源供电，获得一个对地2Vrms电源，以减少外部交流耦合电容及抑制电路。AK4688芯片ADC部分动态范围为99dB，DAC部分动态范围为105dB。设计方案不仅介绍了AK4688芯片ADC及DCA的特性，而且还介绍了该芯片的异步操作特性。同时，本方案提供了一个S/PDIF接口以方便连接数字音频设备。本文详细介绍了AK4688的主要特性、内部框图、应用框图以及采用AK4688芯片设计的评估开发板电路原理图及PCB Layout图。
The AK4688 is a stereo audio CODEC. The integrated ADC and DAC interfaces accept up to 24-bit input/output data and support an asynchronous operation. The input range of the pre-amplifier, that supports line inputs, is adjustable by an external resistor. A ground referenced 2Vrms output with a 3.3V single power supply is achieved by an integrated charge pump, reducing external parts such as AC-coupling capacitors and mute circuits. The ADC block of the AK4688 achieves a dynamic range of 99dB, and the DAC block achieves a dynamic range of 105dB. The AK4688 is suitable for digital recording systems, digital TVs, Blu-ray recorders and Home theater systems.

AK4688主要特性
􀂆Asynchronous ADC/DAC Operation
􀂆Capless Stereo Pre-amplifier for Line Input/Output
􀂆24bit Stereo ADC
- 64x Oversampling
- Sampling Rate up to 48kHz
- Linear Phase Digital Anti-Alias Filter
- S/(N+D): 83dB
- Dynamic Range, S/N: 99dB
- Digital HPF for Offset Cancellation
􀂆24bit Stereo DAC
- 128x Oversampling
- Sampling Rate up to 192kHz
- 24bit 8 times Digital Filter
- S/(N+D): 95dB
- Dynamic Range, S/N: 105dB
- De-emphasis Filter
􀂆High Jitter Tolerance
􀂆External Master Clock Input:
256fs, 384fs, 512fs 768fs (fs=32kHz ∼ 48kHz)
128fs, 192fs, 256fs 384fs (fs=64kHz ∼ 96kHz)
128fs, 192fs (fs=128kHz ~ 192kHz)
􀂆2 Audio Serial I/F (PORT1, PORT2)
- Mster/Slave mode (PORT1)
- I/F format
PORT1, 2: MSB, LSB justified (16/24 bit), I2S
􀂆Hardware / I2C-bus Control
􀂆Operating Voltage:
- Digital I/O and Charge Pump: 3.0V ∼ 3.6V
- ADC Analog: 3.0V ∼ 3.6V
- DAC Analog: 3.0V ∼ 3.6V
􀂆Package: 36pin QFN

图1.内部框图

采用AK4688评估板参考设计
AKD4688-A is an evaluation board for AK4688, which is a single chip 24bit CODEC with a 2ch ADC and a 2ch DAC. It is possible to evaluate not only A/D and D/A of the AK4688, but also asynchronous operation characteristics. Also, the board could be connected to a digital’s audio equipment easily with S/PDIF connectors.

AK4688评估板参考设计的主要特性
􀂆Compatible with 2 types of digital audio interface
- Optical (S/PDIF) input/output with DIT/DIR (AK4118A)
- 10pin headers for interfacing with external data source
􀂆Separate power supply or regulator power supply
􀂆10pin header for register control

图2.参考设计框图

AK4688评估板原理图

图3.参考设计原理图（1）

图4.参考设计原理图（2）

图5.参考设计原理图（3）

图6.参考设计原理图（4）

图7.参考设计原理图（5）

图8.参考设计原理图（6）

AK4688评估板PCB Layout图