This is my first time designing circuit for ADC and I have built a board a while ago but unfortunately did not get good result, and I am pretty sure I made some design error here and there so I just put my idea aside. Now that I got more free time on my hands, I wish to pick up the project again and design a new board (and to restore my pride), I am wondering if you guys could offer any recommendation.
So I got 9 analog sensors (basically a photo-transistor plus a precision resistor) that output from 0-3v3 that I would love to enhance the sampling accuracy, and I have been thinking about using an external ADC to take higher quality samples. I got a couple of LTC1864 16-bit delta sigma dual channel ADC that runs on SPI which made it easy to interface with an Arduino. Since I got 9 sensors, I have also got an analog MUX. I would like to operate my sensors at the same exact voltage as my ADC reference at 3.300V, therefore I also got a LM4132 precision LDO voltage reference chip dropped down from a linear regulator at 5V from a switch regulator from a 12V battery regulated by a solar charged controller. Finally, the voltage reference won’t source enough current to drive all of my sensors, so I included an unity gain OpAmp to amplify the current. I digitized the signal right at the output of the MUX in attempt to minimize noise. As a backup, I also transmit the analog signal to my internal ADC through a coaxial cable just for comparing the result. The shield is grounded only at the sensor side to avoid ground loop. The signal from the photo-transistors is maybe 10Hz at most.
Tech sheets are given below:
Long story short, both 10bit and 16bit results are crappy, with huge amount of fluctuation. It got better after I removed the two OpAmp, but still both result are not acceptable. The trial was done a while ago and I think I have already thrown the data away. But just by looking at the design, what do you guys think might have went wrong? I would love to re design the board, so any comments will be extreamly helpful!