Characteristics of a pH Circuit
A pH probe is a glass or plastic probe that is placed in a solution to measure the pH of that solution. In the aquarium industry, it is common to measure tank pH as well as the pH of some media reactors.
The probes produce a small DC voltage that is proportional to the pH of the solution. The volt reading is quite tiny (theoretically 59.16 mV per pH unit) and is bipolar (theoretically pH 7.0 produces 0 volts output). At first glance, it would appear that all you need is a voltmeter and a conversion chart. However, this is not the case.
Several factors make pH measurement tricky. The first is the source impedance of the pH probe, which typically runs in the 10 MΩ to 1000 MΩ. This extremely high source impedance requires that the input circuit have an even higher input impedance, on the order of 1 TΩ (that is 1 million ohms). This high impedance is very difficult to achieve and results in a circuit that is exceptionally subject to noise from surrounding electrical equipment.
Related to the input impedance is the input bias current. Because the internal source impedance of the probe is so high, the input bias current of the measurement input must be incredibly low; in the order of 100 fA (One Femptoamp is one quadrillionth of an amp). It is hard to conceive of how small of a current this is, but as a reference, one fA is equal to 62,420 electrons going by per second. If you were to put your thumb and index finger on the ends of a AAA battery, the current flowing through your fingers would be 300 million times the amount described here.
In the aquarium industry, pH probes connect using a BNC connector which makes things even worse. BNC connectors are designed for RF signals and not ultra-high impedance DC signals. A better connector would have 3 or 4 contacts and would allow for a balanced pair. However, since this connector has become a standard, it must be worked around.