SBOS925A December 2020 – January 2021 OPA391
First, determine the VREF voltage. This voltage is a compromise between maximum headroom and resolution, as well as allowance for the minimum swing on the CE terminal because the CE terminal generally goes negative in relation to the RE potential as the concentration (sensor current) increases. Bench measurements found the difference between CE and RE to be 180 mV at 300 ppm for this particular sensor. To allow for negative CE swing, footroom, and voltage drop across the 10-kΩ resistor, 300 mV is chosen for VREF.
Next, calculate the maximum sensor current at highest expected concentration:
Then, find the available output swing range greater than the reference voltage available for the measurement:
Finally, calculate the transimpedance resistor (RF) value using the maximum swing and the maximum sensor current: