PCM2706 USB DAC with I2S

Posted on Feb 6, 2014

A couple of USB DACs but no one have I2S output. Some people doesn`t need S/PDIF input and they want only USB. With these requirements is unnecessary to convert signal first from USB to S/PDIF and next to I2S for DAC. This board have analog headphone output, electric galvanically isolated S/PDIF output and optical TOSLINK. After switching jumper circuit switch to I2S mode,

PCM2706 USB DAC with I2S
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which can be directly connected to DAC. Diagram use PCM2706 circuit from Texas Instruments. Circuit includes Digital/Analog converter, S/PDIF output, I2S output and HID interface for push buttons. Schematics diagram is similar to recommended diagram in datasheet. USB input has standard miniUSB connector. Circuit PCM2706 has integrated headphone amplifier and his output goes through external low-pass filter to standard 3. 5" jack socket. Circuit includes digital volume control in a range 0dB to -64dB with 1dB step. This control works only with analog output. It doesn`t have effect for I2S and S/PDIF outputs. In S/PDIF mode (FSEL=1) is active S/PDIF data output and on pins FUNC0 to FUNC3 are inputs for push buttons, which are not used. S/PDIF output goes to optical transmitter and to RS485 driver which boosts signal to voltage divider which divide signal approximately to 1 volt and adjust output impedance to about 75 ohms. Signal is galvanically isolated by pulse transformer. In I2S mode (FSEL=0) goes to the DOUT data in I2S format (16 bits left justified) and on pins FUNC0 to FUNC2 are LRCK, BCK and SYSCK signals. On the pin FUNC3 is now I2S data input signal which is connected to the internal DAC. In my circuit is only on the testpoint and headphone output is silent in this mode. If you want to have active this output in I2S mode, that you can connect it with DOUT. This functionality is described for use with sound effects...

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