Figure 11 shows the control loop th

Figure 11 shows the control loop that represents the operation of the pH-static
enzyme sensor. The pH inside the enzymatic membrane is measured with an ISFET,
resulting in an output voltage Vout. Vset is the desired output voltage that for instance
reflects the original pH of the sample. The error signal ε=Vset -Vout is transformed to a
generating current I by the controller. The current I delivers C equivalents of either H+
or OH-
ions to the membrane, which in turn changes the pH of the membrane in a
ratio depending on its internal buffer capacity. It is obvious from this figure that the
effect of the enzyme reaction is no longer dependent on the buffer. The enzymatic
reaction products are neutralised at the central summing point in the figure, before the
buffer can assert its influence.