While working with Peter from ManyLabs/WireGarden, we wanted to begin to create a system of open source water quality sensors that can be cheaply deployed in schools, remote research, maker projects, basically anywhere there is a need! This project is the result of the knowledge gained from the pH line of interfaces combined with research and prototyping, I spent a lot of time incorporating a solid analog font end to interface with most eC probes with minimal current and disruption to the Solution under test.
By using a cheap, accurate and reliable I2C ADC we can both increase accuracy and decrease cost, and also create a product that people can source parts for globally at reasonable prices. It also makes the unit monolithic in the fact that it it all that is needed to interface an eC probe and therefore take readings! while the the ADC is in a small package they are easy enough to handle even for hand soldering! This unit can even be hand assembled.
The analog front end is broken up into 4 main parts (well 3 really). The first part is a Wein Bridge Oscillator to create a low distortion sine wave. Right before the 2nd stage, this wave is turned into a small signal(about .2VPP via voltage divider) this small signal is passed into the (2nd stage) gain stage where the eC probe forms one leg of the gain divider of the op-amp. By treating the SUT as an unknown resistance in a gain loop the amount of current needed to calculate the conductivity is decreased by several factors(at least a 100x reduction). The 3rd stage is a super diode precision rectifier which then goes into a buffer to tie off the last op-amp in the quad package.
For more detail: MinieC I2C eC interface
