AN4348 datasheet(11/25 Pages) STMICROELECTRONICS

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AN4348

Galvanic configuration

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Galvanic configuration

If we consider an O2 sensor used to detect under- or over-exposure to O2 in a "breathable

environment", the current generated by the sensor is hundreds of µA. In this case, the

TSV711 (9 µA) is a good candidate in the application because the current level is big

enough.

An alternative configuration is to use the voltage drop through the load resistor and amplify

this voltage with a suitable op-amp. The current to voltage conversion is made by RL rather

than by the trans-impedance amplifier (see Figure 8).

Figure 8: Low consumption galvanic oxygen sensor configuration

The scale of the signal to be amplified is then relatively low (RL×Isense is in the range mV).

For this configuration, the op-amp has to be a precision one, and above all, its input offset

temperature derating has to be very good. Indeed, input offset voltage compensation

should be possible, but temperature compensation may be harder to achieve. The best op-

amp choice for this application is the TSZ121 which is a chopper op-amp. Its specifications

are 0.06 µV/°C maximum and 5 µV maximum. Therefore, a 20 °C variation causes a

variation of only 1.2 µV which corresponds well with the accuracy required by the

application. With this configuration a good accuracy is reached (1 µV is only 0.01 % of a full

scale 10 mV).

Another advantage of this configuration is that even if the power supply is turned off, the

sensor is kept biased thanks to RL. Consumption of the application can be reduced by

powering it periodically (keeping in mind that the capacitors have to be charged at each

cycle).

The output signal read by the ADC is: - (1 + (Rf/Rg))IsenseRL

Note that Isense is negative (as we are dealing with O2), so this voltage is positive.

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