Easy On The Sound-Card, Extra Capacitors

This project actually got off to a slow start. It started from a simple idea of making a high-end Digital To Analogue (DAC) converter, more commonly known as a sound-card. So the research began in three main websites:

  1. PupDAC project
  2. Pavouk’s website
  3. DrDAC project

All of these projects have a lot of insights regarding which Integrated Circuits (ICs) should be chosen, what amplifiers to use and what not. Most of them utilize at least one of TIs PCM270x Audio USB converters and (surprisingly) one of TIs PCM179x DACs. The latter series has some very good components, with DACs that can support up to 24 bit resolution depth and up to 192KHz sample rates, whereas CD quality is considered 44.1KHz.

However the USB to Audio converters all have one thing in common: They are limited to 16 bit depth. As far as my search went, there are barely off-the-shelf ICs that provide 24 bit resolution, which makes it pointless to kick-off a project that is supposed to give you a really high-end DAC in the end, no? So wandering mindlessly around, I ran into this chip – CM102s, that also gives a 16 bit resolution, however with a low sample rate of up to 48KHz. I’m guessing that the internal DAC supplied is of much lower quality, but the sheer simplicity of the component was enough to encourage my interest.

Only 18 pins to take care of? Count me in! Who cares about the lack of differential audio outputs, 0 to 5 swing and the unknown terrifying quality of the internal Op-Amp! This might actually be fun. So, as usual, one begins by reading the datasheet and connecting the pins accordingly.

I have to say, relative to other datasheets I have seen this one is very lacking, however it does supply enough information.

  • #1 PA (Power Amplifier) Enable pin – It is stated that this pin enables a power amplifier, but the output impedance is not supplied. It does say it is supposed to drive speakers, so it can be assumed that it supplies relatively high output impedance. In any case, the schematic\layout can be made you can choose.
  • #2 GPIO – Other than this is used according to the vender (mistake in origin), it is unknown what this does. So this is pulled down.
  • #3-4 XI, connected to a 12MHz crystal, for external clock.
  • #5 DVDD5V – Digital Vdd 5V supply (input).
  • #6 REGV – Regulator Voltage, 3.3V (output).
  • #7-8 USBDP\M – USB differential pair. D+ needs to be pulled up to 3.3V. Customary this Pull-up resistor needs to be connected using a 1-1.5K resistor. Since I already ordered a 2K resistor for the LED, I used that. Hopefully that will be OK. Matching resistors are used on these pins, also. In a design I found on the internet, 22Ohm resistors were used.
  • #9 DVSS – Digital Vss, grounded.
  • #10 TEST – Test mode switch. Pull this down.
  • #11 VREF – This is for the reference voltage used in the DAC. Since the swing this DAC provides is 0-5V, the reference voltage is 2.5V. The reason a capacitor is used here will be explained in a moment, however this needs to be a relatively large capacitor.
  • #12 VOLADJ – VOLume ADJust pin. 0-5V input, where 0 is 0 volume and 5 is maximum volume.
  • #13 AVDD – Analog Vdd 5V (Input). Shouldn’t this be referred to as Vcc?
  • #14-15 LOL\R – Left\Right audio output.
  • #16 AVSS – Analog Vss. Grounded (Again, Vee…).
  • #17 SPDIFO – S\PDIF Output. I dumped this to the ground via 10K resistor, but this can be used to extract the S\PDIF digital signal.
  • #18 PDSW – Power Down SWitch indicator. This is logical ‘1’ (3.3V) when the sound card is not in suspend mode. I connected this to a pretty blue LED.

That was fast! So lets go over the layout for this one.

Well well… let’s cover the easy ground, first.

9; 16: Grounds are connected to the ground. 5-6; 13: All power pins, 3.3V and 5V are decoupled both for digital signals using a ceramic SMD capacitor and for analog signals using 10uF electrolytic capacitors. 18: LEDzzzzz OMG pretty light!!

This is a standard LED driver circuit, using an N-Channel MOSFET. I used 2N7002, simply because it’s available.

3-4: Connected to a 12MHz oscillator via 22pF capacitors. 11: Connected via a 100uF capacitor to the ground. This, I assume, is to allow a “escape path” for the small signal to the ground. This method is also used in single rail audio amplifiers, so the AC signal has a “solid” ground to refer to. 7-8: The USB D+ pin was pulled up to 3.3V using a 2KOhm resistor

In general, every power pin was decoupled. In the next post, I’ll lay this out, but the capacitors chosen were 10uF electrolytic and 470nF ceramic SMD, because it was the largest I could find on the website. This decoupling reduces noise in the power supply of the IC, which generally quites everything down.

The Op-Amp of choice here is a NJM4556, because it can operate in relatively low voltages. I laid it out in a single rail supply.

Have a blast!

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