i8

New audio ADC PCB

Schematic

PCB

After a very long week-end of work, I finally have a fully symmetric version of our audio ADC. Not only that, but I managed to cram low dropout voltage regulators to go from 18 V to 15 V, which will provide a stable power source for the four operational amplifiers. Now, the left and right bias lanes have their own regulators, and so do the left and right digital sections. This brings the total number of embedded regulators to eight.

The PCB shown above does not have any ground plane yet, but it will be added later this week. Nevertheless, I made sure that the two regulators used for the digital section could be powered through the digital section of the ground plane. I also left some room for two pairs of connectors that could be mounted on the PCB in order to feed the analog audio inputs, thereby bypassing the card edge connector for the most important signals. Doing so, the card edge connector would only be used for digital signals, clock signals, and power. In fact, I’m even thinking about doing something similar for the master clock signal. Now, I need to find the right SMD connectors for a pair of balanced inputs.

I also made sure to use larger traces whenever possible. This should help reduce impedance as much as possible. This is especially obvious when looking at the two pairs of left and right horizontal lanes visible on the upper part of the top layer (red) and used to carry the analog audio inputs: as straight and fat as can be…

Last but not least, I made sure that the bottom plane would be used only for grounding and short and (mostly) straight power lanes, at the exception of one low frequency analog signal, HPFE, which is used to enable the high pass filter. Routing this signal through the bottom plane for a few millimeters allowed me to make the design totally symmetrical, thereby saving up to 10 mm of length on the board. Everything else takes place on the top plane.

Coming up with such a tight design was a lot harder than I thought. But it’s not totally surprising when you realize than the board will be 100 × 40 mm, which is tiny considering the number of components that it must house, while using two conductive layers only.

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