There are many mixed analog/digital circuits today. Often the components like ADCs, DACs or codecs have separate power supply pins for analog and digital parts of the circuit. However, most modern opamp-based circuits have a very high power supply rejection ratio (PSSR). Does it really make sense to use separate power supplies for the analog and the digital part?
Let’s have a look at a specific example – our HiFiBerry USB. The PCM2906C from Texas Instruments used in this design can be powered completely from the USB bus. It has internal voltage regulators the create a 3.3V voltage supply from the USB bus voltage. But if you look in the datasheet, you will notice that Texas Instruments recommends an external REG-103 voltage regulator for high-quality audio.
Is there really a difference? Let’s see. For our tests we measure harmonic distortions of both the input and the output of this codec. Input and output are connected by a simple loopback cable. Input and output run at full swing (about 2Vpp).
We will start without an external voltage regulator. The chip is powered directly from the USB bus power.
0.01% harmonic distortion (D2) are not too bad. This chip is not the best codec available on the market. However, there is a lot of D6+ distortion peaking to 0.1%. That doesn’t look good. Where does it come from? Let’s see, what happens if we use a REG-103 voltage regulator for the analog supply of the circuit:
Much better! D2 and D3 are at about 0.005%, D4+ even much lower. This is a really nice-performing codec now.
There is also another interesting fact here: Even with the additional voltage regulator, the whole circuit is still powered via the USB bus voltage. With a high-quality voltage regulator there is no need for an external power supply, which is good news.