Antilog converter linearizes carbon dioxide sensor
While most carbon dioxide sensors use IR technology, electrochemical sensors are a serious competitor because of their high sensitivity, wide measurement range, and low price. As a rule, electrochemical sensors connect to a microcontroller through a buffer amplifier with an extremely low bias current (<1pA). The micro is needed to linearize the logarithmic response of the sensor. A good example of this approach is the SEN-000007 module from Sandbox Electronics, which uses an MG-811 CO2 sensor from Hanwei Electronics. Reference 1 reveals the circuits and the code, but does not specify accuracy.
This Design Idea shows a pure hardware solution to the linearization problem with a simple and cheap circuit that features easy adjustment and good accuracy. The output signal can go directly to a panel meter or a microcontroller, requiring no complex data manipulation such as log or antilog calculations.
The MG-811 sensor measures CO2 concentration from 400 to 10,000 ppm (0.04% to 1%). Figure 1 displays the transfer function of the sensor. The data points come from the datasheet of the MG-811. The line and equation are generated by the curve fitting tool of Microsoft Excel.
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