Spent most of the day today playing with the pre-amplifier design. The design uses the same instrumentation amplifier chips (ina2332) for both large and small electrodes, however, this becomes problematic. The ina2332 is a dual amp chip, where each instrumentation amplifier is based on the two-op-amp design and is physically small, so there are worse parasitics when compared to larger three-op-amp designs. One of these parasitics is input bias current, which in this case is 1-10pA. Sourcing this kind of current is no problem for the larger electrodes, but has become problematic with the micro-meter sized electrodes. As a result, I designed a module to use two ina116 chips in place of one of the ina2332. These chips are single channels and are a few times larger than the ina2332s, but, they are less noisy and require only 10fA of input bias current. By making a few estimations and writing out the electrochemistry I-V relationships, one can attribute 100-400uV of voltage noise to the side-effects of a 10pA input bias current and how they are more manageable with a 10fA bias current. The lesson for today is that 10^-12 is small, but not insignificant.

 
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