where pHcal is the pH of a calibration liquid at 37°C(Tcal), Vgs the electrical output
signal of the ISFET amplifier circuit and S the pH sensitivity (mV/pH) of the
particular ISFET, stored in the memory. Although the ISFET is roughly applied at the
temperature-insensitive bias point, it appears that a temperature sensitivity of Vgs as
well as S still occurs, which has to be corrected. Furthermore it appears that, after an
initial drift, the time drift of the A12O3-ISFET can be considered to be linear for a
certain time. Therefore a linear time drift correction is necessary. This correction (DC
in mV/h) as well as the temperature coefficient TC (in mV/°C) and the temperature
dependence of S, dS/dT (in mV/pH/°C), are also determined in vitro and stored in the
ISFET-PROM connector. These data are used for the exact pH determination by the
signal conditioner according to: