Each ISFET was factory tested in vitro before sterilisation, and the essential parameters, which will be dealt with below, are stored in a PROM (Programmable Read Only Memory) which is an integral part of the ISFET connector. Experience obtained from animal as well as clinical experiments has shown that the in vitro characterisation is also valid for the in vivo use of the devices. Therefore, the data stored in the PROM connector also have an in vivo valuation and are used by the floating signal conditioner to convert the ISFET output signal to the blood pH.
The signal conditioner applies the ISFET in the constant drain current mode, with constant drain-source voltage, Vds, resulting in a gate-source voltage, Vgs, which directly reflects the pH-sensitive interfacial potential at the gate surface.
If no temperature sensitivity and time drift would occur, the equation handled by the signal conditioner can be simply: