pH sensor, is the cebo-lc device suitable?

Question

I would like to interface a pH probe. I know there are various threads on this topic. If found this two most interesting:

Miceuz suggests two suitable instrumentation amplifiers (INA 116 and AD549). In this post markrages suggests in one of his comments to just use single ended amplification if there is not a lot of noise due to common mode voltage. However, he also stresses that simple instrumentation amplifiers would not work here.

The cebo-lc has an AD8692 with an input bias current between 1 and 50 pA. That is far more than for e.g INA116 (10 fA). Is the CEBO-LC therefore unsuited for this task ?

Answer 1

A typical glass pH probe has an impedance in the 10's or 100's of M\$\Omega\$. The sensitivity at room temperature is around 60mV/pH. So if you want the error due to op-amp bias current to be less than 0.01pH and the probe happens to be 100M\$\Omega\$ then the maximum bias current is 0.6mV/100M\$\Omega\$ = 6pA.

Keep in mind that those are nominal values and the output changes greatly with temperature and op-amp bias current changes greatly with temperature. The 50pA you mention is for temperatures up to 85°C, at room temperature it's maximum 1pA (typical 200fA) but at 125°C it can be as much as 260pA. The LMC6041 is guaranteed to be less than 4pA. The INA116 (depending on type) is guaranteed to be less than 0.025pA at 85°C, quite impressive.

Low offset drift is usually of less importance due to the relatively high output voltage.

So the AD8692 is not really unsuitable in all cases, but may not be the best choice. Certainly, if I was designing a general purpose high end pH meter I would aim for lower bias current. It's probably overkill for a cheap pocket pH meter- more likely a CMOS-input single-ended amplifier would be used.