Now that we have opted to use R-2R sign magnitude modulators for our audio outputs, PCB real estate will be at a premium. In such a context, we have decided to mount our audio ADC and DAM sections on individual modules, using the 100-pin SO-DIMM form factor. With such an approach, every stereo audio input will have its own SO-DIMM module (with one AK5397EQ on it), and so will every mono audio output.
100-pin SO-DIMM modules measure 90.17 × 25.4 mm, which is plenty enough to put what we need on them. For the audio ADC, it will be the AK5397EQ converter and six operational amplifiers on one side, and a ground plane on the other. A full system will have 8 of these modules. For the audio DAM, it will be over 100 resistors and 4 shift registers on each side of the board, and a ground plane in the middle. A full system will have 16 of these modules.
Because a SO-DIMM board does not have any connector and is really small, it is actually really cheap to manufacture. Furthermore, our ADC and DAM modules should only contain surface-mount devices, which should further reduce manufacturing costs. And from a backplane standpoint, all we will need is a TE 5390213-1, which allows the SO-DIMM module to be mounted vertically, thereby reducing the use of PCB real estate to the absolute minimum, for a cost of $2.31 per module.
The SO-DIMM connector is 7.54 mm wide. If we were to mount it with 1.27 mm between two connectors, we would need 70.48 mm of width for the 8 ADC modules, and 140.96 mm for the 16 DAM modules. This would leave about 145 mm of width for the power supply unit, which is not too bad.
Thanks to this approach, the most expensive parts of our design would be mounted on 24 different modules, which could be purchased incrementally. For example, an initial setup could contain a single stereo input, and a single stereo output, which would approximately cut the initial cost of the device in half.
This approach would also make it easier to upgrade the system over time, and to perform A/B testing for different ADC modules or different DAM modules. And during the development phase, it would dramatically reduce the cost of iterating through multiple versions of PCB designs.