Switched Flip-Flop based Preprocessing Circuit for ISFETs

In this paper, a preprocessing circuit for ISFETs (Ion-sensitive field-effect transistors) to measure hydrogen-ion concentration in electrolyte is presented. A modified flip-flop is the main part of the circuit. The modification consists in replacing the standard transistors by ISFETs and periodically switching the supply voltage on and off. Concentration of hydrogen ions to be measured discontinues the flip-flop value symmetry, which means that by switching the supply voltage on the flip-flop goes to one of two stable states, ‘one’ or ‘zero’. The recovery of the value symmetry can be achieved by changing a balanced voltage, which is incorporated to the flip-flop, to bring the flip-flop to a 50% position (probability of ‘one’ equals to probability of ‘zero’). Thus, the balanced voltage reflects the measured concentration of hydrogen ions. Its magnitude is set automatically by using a feedback circuit whose input is connected to the flip-flop output. The preprocessing circuit, as the whole, is the well-known ∆ modulator in which the switched flip-flop serves as a comparator and a sampling circuit. The advantages of this approach in comparison to those of standard approaches are discussed. Finally, theoretical results are verified by simulations with TSPICE and a good agreement is reported.