Plan for reading Carbon Monoxide sensor?

Other Hardware Sensors
1

I would like to read carbon monoxide values from a 2ECOH sensor for a project that I am working on, with an ESP32.

At first I thought that I could just use the built-in ADC, but my current understanding now is that it will not work due to the sensor's output in nano amps. The manufacturer actually provides a recommended circuit: http://www.ddscientific.com/uploads/5/7/1/3/57136893/2eco.pdf

I am trying to make sense of the circuit above but having a hard time understanding. What is VRef in the circuit? Am I supposed to be supplying it, or should I plan to use it for the ADC? Additionally, I don't understand why the sensor has 3 inputs, when it only has 2 electrodes.

Any guidance here would be helpful. I would love to be able to interface with this exact sensor. Thanks!

2

Technically, that extremely high impedance sensor is difficult to work with, especially for an amateur.

If you want to build your own interface, you have little choice but to duplicate that circuit, on a professionally designed PCB with good low leakage characteristics, or buy a commercial version of it.

As you point out, some important details are not included in the schematic, so you will need to contact the manufacturer for more information.