Figure 7-4 shows a simple micropower potentiostat circuit for use with three-terminal unbiased CO sensors. This same design is applicable to many other type of three-terminal gas sensors or electrochemical cells. The basic sensor has three electrodes: the sense or working electrode (WE), counter electrode (CE), and reference electrode (RE). A current flows between CE and WE proportional to the detected concentration. The RE monitors the potential of the internal reference point. For an unbiased sensor, the WE and RE must be maintained at the same potential by adjusting the bias on CE. Through the potentiostat circuit formed by U1, the servo feedback action maintains the RE pin at a potential set by V_REF. R1 maintains stability as a result of the large capacitance of the sensor. C1 and R2 form the potentiostat integrator and set the feedback time constant. U2 forms a transimpedance amplifier (TIA) to convert the resulting sensor current into a proportional voltage. Equation 1 calculates the transimpedance gain, and resulting sensitivity, using R_F:

R_Load is a load resistor with a value that is normally specified by the sensor manufacturer (typically, 10 Ω). The potential at WE is set by the applied V_REF.

Figure 7-1 Three-Terminal CO Gas Sensor