AN4348

Signal conditioning with a three-electrode sensor

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Signal conditioning with a three-electrode sensor

To use a three-electrode sensor, a voltage has to be applied between the WE and the RE

according to the specification of the sensor. The current generated on the WE has to be

balanced by the electronics on the CE. No current should flow through the RE. A trans-

impedance amplifier configuration is used to convert the current generated by the sensor

into a voltage that can be read by the ADC of a microcontroller. This whole electronic

design is called a potentiostat. Sensors are generally specified for a given resistive load

which must be "seen"

by the sensor. This value is generally in the range 10 Ω to 100 Ω.

Figure 4 exhibits the main parts of the potentiostat circuitry. The U1 op-amp converts the

can be either positive or negative (see Figure 2 and Figure 3). U1 also ensures that the

also ensures that no current flows through the RE while providing the right amount of

current to the CE to compensate the one on the WE.

The microcontroller then computes the gas concentration from the reading made by its

Figure 4: Potentiostat principle