HD Audio from ground up (Part 2)

While in the previous post we describe the main idea of the HD Audio together with the DAC selection and the first considerations, now it is time to move to the actual design.

We are going to use the Design Spark PCB for the schematic capture and the creation of the PCB layout. Since I haven’t use this software before I scheduled a fair amount of time just to learn the Design Spark PCB process. This turned out to be unnecessary, the learning curve is smooth and gentle, so you can actually start building real professional designs in just a few hours.

Library creation and Schematic

Before we can begin with the schematic, we must first build the component libraries that we are going to use. Those are the DAC (RS Part No: 662-5021), the external oscillator (RS Part No: 707-9747), and the audio in and out socket jacks (RS Part No: 705-1490).

The process of a new library creation is easy and if you follow the step by step process, described at the given pdf (located in the Doc folder inside the DesignSpark installation) you will have an accurate representation of the desired footprint. Of course if you haven’t created any schematic/PCB library before then you will probably meet a few problems. Here are a few of my tips that you can follow in order to get you project off the ground.

1. Make your firsts schematic symbols based on an already made component from the given libraries. For example you can copy-paste one schematic symbol from an existing library and make changes there. After you gain experience you can create your own symbols from the beginning.
2. For the footprint, set the grid to be equal to the chip pin pitch (or sometimes half of that). It helps to align the pads really quickly and without errors. After that you can change the grid size accordingly, for the creation of the silkscreen etc.
3. Use both mm and mils. As older components used imperial dimensions and the newer components use metric, it is good to familiarise yourself with both units, and how to convert between them.
4.  Always check the mechanical drawings of the datasheet and if you cannot find a rounded value in mm then there is always a rounded value in mils and vice versa. For example don’t try to make a grid 1.27mm. Change the grid units to mils and set the value to 50mils. Also it is good to know that 100mil = 2.54mm and 200mil = 5.08mm
5. Avoid confusion of different units (thou, mil, decimal point units). Just use the mil units, and know that 1mil = 1thou = 1point = 0.001inch.

Having those tips in mind the library creation is easy. After that follows the design of the schematic itself (Figure 1). Remember to include a frame before you place any components at the schematic. Furthermore, always have the datasheet and your notes with you. After you finish the design check, thoroughly review your schematic before you move to the PCB layout. Mistakes made in the schematic can be rectified easily, but once the PCB is laid out, changes become harder to make.

Moving from the general guidelines it is time for some notes about our actual design.

The 12MHz oscillator in the SMT package is located at the top right part of the schematic. The output is constantly on (as we hard wire the EN pin to the positive voltage) but it is possible to route the EN pin to an external IO pin in order to control the output of the oscillator (in order to reduce the power consumption for example). The 12MHz clock is needed when the DAC operates as a master, and for this design this is the main mode we will use.

Below the oscillator schematic we will find the audio in and out connectors. It is important to have the peripheral components (like the resistors and the capacitors) close by. The careful placement of those components together with the proper values will produce a better frequency response. It is desirable to have even amplitude across the entire audio frequency spectrum.

At the top left we have the decoupling capacitors for the voltage pins of the DAC and the pull up resistor that controls the mode pin. This pin is routed to the external pin header and indicates if the internal registers of the DAC will controlled over the SPI (mode pin = logic 1) or I2C (mode pin = logic 0). We tested both the SPI and I2C modes, but we are going to discuss that more Part 3.

Finally at the middle there is the TLV320AIC23B itself with only two peripheral components. One 10uF electrolytic and one 100nF ceramic capacitors in parallel. Both of those are highly important for the stability of the internal midrail voltage of the DAC and should be placed as close as possible to the Vmid pin of the DAC. The placement of two different value capacitors in parallel is a common technique for sensitive voltage rails. The first one is working better at the lower frequencies while the other is working at the middle ends. For example is it common for higher frequencies to have and 1nF caps. 

PCB Layout

The translation from the schematic to the PCB is easy. From the menu bar of the Design Spark we select “Tools” and then “Translate to PCB”. The software will automatically load the layout editor and all the components together with the appropriate net list on the right. At this point we can start laying out the PCB, and so it is important to ensure that the schematics are 100% correct at this stage. Of course you can make schematic changes later and forward the changes to the PCB and vice versa but sometimes creates confusion especially if you work with high complexity designs.

The design for our project is simple with only a low count of components and two main constrains. The final board must be one component layer (two cooper layers) and as small as possible.

After a some testing with different component orientations in order to meet the constrains, the result is below (Figure 2). The blue colour is a cooper fill that carries the GND. At the right we have the audio in/out jack sockets and at the left the pin header for the communication with the mbed (or any other microcontroller in general).

The next step was the creation of the gerber files. Having active the PCB editor, we select from the menu bar “Output” and then “Manufacturing Plots...” (or Shift+P if you used to work with keyboard shortcuts). A new window will appear and after a few clicks we have the gerber files for the layers we want.

Below is the final result! The HD Audio module produced by a local professional PCB manufacture shop and hand populated internally.  

Next week we will start looking how to create a software library to drive this DAC

For the full design process: http://mbed.org/cookbook/HQ-Audio

For the Part 1: http://www.designspark.com/content/hd-audio-ground-part-1