The schematics and the PCB design of the first version of the logger -- the home etched one with an XMega controller -- can be reviewed by downloading the above link. It was all made using Altium Designer, with a little bit of inspiration from the Atmel Xplained design documents.
One thing I ought to mention is that there are hardware bugs in this design.
- The resistors, or lack of, between the electrodes and the input of the instrumentation amplifier. They should be there to protect the amplifier from ESD.
- The device was meant to be switched on with a soft switch, but the button is not connected to an asynchronous pin on the controller.
- The ADC is fed the same voltage as the rest of the microcontroller, which according to the datasheet is 0.6V or so beyond the maximum allowed.