Changed the LT5400 footprints to exposed pad, I have also exposed a mask area to heat with a soldering iron to melt under the pad if you really are limited to an iron, If you had to use this method, I would recommend getting the pad hot, apply some solder to the pad, clean it back with some wick and try and hold it molten, and only then tweezer in the part,
Buffer is variable heating so keep away from the temperature sensitive parts, check, Its part of why I went a big overkill on the array thermal situation, the vias and copper will reduce the amplitude of any thermal gradients, but I will shift it around,
The input connector is the star grounding point for all the measurement side of things because it is that point that all measurements to the outside world are relative to, this was part of why I was going a bit out of my way to keep certain parts of the grounding seperated (I went well overkill in that respect and can lower it to 4 connections) So my thinking was, if any variable current is sent down a shared connection back to that master ground node, you get a measurement offset or noise. To this end I was thinking of changing it to a 2x05 Connector to give each external signal its own ground connection.
That ground point is then connected back to the power supply like a more conventional ground star point because any offsets there do not alter the measured voltage, everything is relative to that point.
R61 falls under that point, But if you want I can route him back to the power supply star point,
I am building the circuit up in easy to rearrange chunks, anything with a silkscreen square and label is what I would call "Locked down" as in, I'm happy with it and should not need to do any major changes to, just possibly shift the chunk around. At present I have not done any pin swaps, and for the analog mux's I'm happy that I will not need to,
The AVR is a bit uglier at present but that is more to do with where the ADC pin is, and no matter how I swap it is not going to help it, the digital outputs are actually ideally positioned and ordered, all grouped at the edge of the micro, routing out the spare pins will also be a bit awkward, For the DAC, I would recommend using 14, 15 or 16 for its chip select
I'm laying out function blocks that I can slide about with very little change to the interconnecting wires, e.g the buffer is only 5 connections out of the block, If that bias was to +15V, it would be 4, so dragging over the entire block and moving it does not cost me much in terms of layout, its more the building of those blocks that take more time. This is why changing things is not hard, even for bigger changes it is just shift the block off the board, make the changes to that block then slide it back in,
There is lots of room left, We agreed to a max of 100x100 just to support almost all dirt cheap PCB suppliers, If you need more room for the Input, I'll just rotate the reference, input mux and buffer clockwise 90 degrees, should have shown the entire board, but space is not at any kind of premium, So throw down any options you want to cater to, The +7V signal to the mux already has 100nF tied to it, so it would appear to be much more immune to charge injection than any other signal.
The UART will end up very close to the Power supply at the moment, likely going to be towards the bottom left of the PCB, I was intending it to just share the AVR ground and 5V, but if you want it run seperatly no issue, was just thinking to close the loop, (The micro is sinking the TX current) As its an isolated interface all I need to do is prevent any capacitive coupling, I was thinking to flip it to the attached image style to make it more in line with normal UART,
Edit: for AVR pin PB2, I would recommend leaving that disconnected or only an output, if he changes state during an SPI transfer it kicks the SPI over to slave mode.