Modding Arduino UNO for Lower Noise
One of the main differences between the Arduino UNO and previous implementations of Arduino, is the 3.3V supply. In UNO, it is provided by a low noise, low drop regulator. In previous implementations it is taken from the internal 3.3V regulator of the USB FTDI chip. There is no specification on the 3.3V supply for the FTDI chip, but the specification for the LP2985 shows that the noise level is in the 30 uV RMS if proper capacitors are used. Another big difference is the supply current: 50 mA for the old boards, 150 mA for the new boards (even though the Arduino UNO specification says 50 mA)
When dealing with audio applications it is good to lower the noise of all the the power supplies and devices involved in the system. In the case of Arduino, it is connected to the DAC through the two I2C lines and the GND connection. For the implementation described here, the I2C lines are enabled by the level converter which is connected (on the low voltage side) to the Arduino UNO 3.3V regulator.
If we compare the implementation of the LP2985 regulator in the Arduino UNO schematic with the recommended implementation is the LP2985 data sheet, we find that the bypass capacitor is missing and the output capacitor is smaller than the size recommended for low noise operation. The diagram below shows these differences. (See data sheet on type of capacitors recommended)
H i F i D U I N O 
Do you have to connect DGND and Audio GND at some point?
Yes but you don’t have to worry about it. In the Buffalo DAC you connect the GND pin for the I2C connection to the GND pin of Arduino. I believe all GND lines merge at the VDD input of the Buffalo board.
I notice you don’t mention the 110KHz noise generated by the FTDI chip on the 3.3V which thanks to it being replaced by the LP2985 is not there on the Uno. Have you actually looked at the output of the FTDI power on a scope?
Hi, Interesting to know. I didn’t know that. So that makes it more “suitable” for audio applications… Thanks.
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