AN4348 datasheet(9/25 Pages) STMICROELECTRONICS

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AN4348

From sensor specification to functional application

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From sensor specification to functional application

From the sensor specification, the minimum and maximum sensitivity (S) is known and is

generally expressed in nA/ppm. However, in the case of O2 the output signal in air (in µA)

is usually reported. The output signal in air is for 20.9 % O2. So, in this case, to find the

sensitivity of the sensor, divide by 20.9 % and express the result in µA/% of O2.

The range of the sensor ([gas]max) is another important parameter which is expressed in

ppm for CO or % for O2. If the whole range is required for your application, the output

voltage on the ADC is between Vref2 (where there is no gas) and

Vref2 ± RT × Smax × [gas]max.

Be careful with the polarity of the sensor. Isense can be either positive or negative so,

the "±" sign is "+" for CO but "-" for O2.

The RT value should be maximized to have the widest possible range at ADC level. But, it

should also be kept low enough to avoid saturating U1 (keeping some headroom in case of

over-range). This ensures the highest possible resolution.

The recommended load resistor of the sensor, specified in its datasheet, gives the value

RL.

RL is a tradeoff between response time and noise. Indeed, any noise on the WE node is

amplified by the op-amp with a gain of 1 + (RT/RL).

Due to its capacitive behavior, the output signal converges with a time constant Cs(Rs+RL),

assuming that the sensor is a capacitor, Cs, in series with a resistor, Rs.

Also, RL should not be too high or it may unbias the sensor (because of an RL×Isense voltage

drop).

AN4348 Datasheet(PDF) 9 Page - STMicroelectronics