Second Generation Sensor Interface Board

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Adrian
Second Generation Sensor Interface Board

I'm starting a thread for info & suggestions for the forthcoming redesign of the sensor PCB. Suggestions welcome, this will not be the ultimate design but hopefully we can have fabbed on a larger scale.
One of the attributes to is monitor weight from one to four hives (or pallets) from one Pi sensor interface PCB.
Due to the complexity, density, and other constraints SMT parts will be employed. This will mean that the layperson will not be able to assemble the board.
The 6p6c telco sensor connectors will very likely be assembled on a daughter board so that they can be mounted through chassis and away from the Pi. This greatly simplifies sensor connections. This also means that to maintain watertight and prevent corrosion silicon gel will be required to prevent moisture damage where connections may be exposed. The lack of sensor cable shielding is a potential weakness. Alternative watertight connectors and shielded cable is available but would likely drive the cost up significantly. We are very focused on providing an affordable system. The cost of every component is considered against the benefit it may yield. That said we are still open to suggestions, nothing is set in stone.
The load cells are currently summed into four 6p6c jacks. Employing the Gen 2 daughter board it will be possible to sum 4 load cells remotely then run one phone cable to the Pi daughter board, replicate four times to monitor 4 hives. This will be possible because we will be utilizing four differential analog inputs on the ADC. Long term it may be possible to monitor eight hives or other analog parameters with two ADC chips.

Selecting candidate ADC chips has been challenging, I have focused on the SPI serial interface to the Pi as it is supported by code and many devices, 24 bits, PGA. Here is a shortlist;
TI ADS1243, $6-$9 for small quantities, code proven with the Pi, http://www.ti.com.cn/cn/lit/ds/symlink/ads1243.pdf
TI ADS1256, $12-$15 small quantities, code proven with Pi, widely supported, http://www.ti.com/lit/ds/symlink/ads1256.pdf
Pi daughter board designed by http://www.waveshare.com/wiki/High-Precision_AD/DA_Board
Evaluation board available on Ebay for $31.34 http://www.ebay.com/itm/Raspberry-Pi-High-Precision-ADS1256-AD-DAC8552-D...
Analog devices and Cirrus logic both offer an abundance of 24 bit SPI ADC chips however no reference projects or code was found.
I don't know if the lack of 128X gain on the ADS1256 is an issue, the ADS1243 has X128, the ADS1256 has many more features, filter, buffer, I/O etc but costs a few $$ more. the ADS1258 is a 8 diff channel version at 2X cost. At this point I feel the ADS1256 is one we should be sampling. Bear in mind the Pi's I2S bus is reserved for high speed analog, microphone, vibration, audio or high speed analog out.
All ideas are welcome,
Adrian