We present the simulation results for a Pulse Width Modulation (PWM) circuit (to be fabricated in a 0.5 μm CMOS process) used to cycle the electric field in an Ion Sensitive Field Effect Transistor (ISFET). Vertical electric field, which controls the inversion layer in a field effect transistors, can be used to reset the inherent drift behavior of ISFET. A PWM circuit, to cycle the vertical field, enables us to precisely monitor the pH of an electrolyte without needing to manually calibrate the ISFET. Two or more ISFETs could be used with the devices alternatively being placed in reset and measurement mode. By combining the outputs from measurement phase of the devices, we can read the pH of the electrolyte continuously. The PWM circuit is composed of a 10.9 kHz ring oscillator, five divider circuits giving a 100,000 frequency division, a 6 bit counter, and a Digital to Analog Converter (DAC) that feeds into a comparator whose output selects the mode of operation for the ISFETs.