-

+

VOUT

V

-

V

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99 k:

1 k:

1 k:

RL

OXYGEN SENSOR

LMP2231

SNOSB01E – JANUARY 2008 – REVISED MARCH 2013

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PORTABLE GAS DETECTION SENSOR

Gas sensors are used in many different industrial and medical applications. They generate a current which is

proportional to the percentage of a particular gas sensed in an air sample. This current goes through a load

resistor and the resulting voltage drop is measured. Depending on the sensed gas and sensitivity of the sensor,

the output current can be in the order of tens of microamperes to a few milliamperes. Gas sensor datasheets

often specify a recommended load resistor value or they suggest a range of load resistors to choose from.

Oxygen sensors are used when air quality or oxygen delivered to a patient needs to be monitored. Fresh air

contains 20.9% oxygen. Air samples containing less than 18% oxygen are considered dangerous. Oxygen

sensors are also used in industrial applications where the environment must lack oxygen. An example is when

food is vacuum packed. There are two main categories of oxygen sensors, those which sense oxygen when it is

abundantly present (i.e. in air or near an oxygen tank) and those which detect traces of oxygen in ppm.

Figure 53 shows a typical circuit used to amplify the output of an oxygen detector. The LMP2231 makes an

excellent choice for this application as it only draws 10 µA of current and operates on supply voltages down to

1.8V. This application detects oxygen in air. The oxygen sensor outputs a known current through the load

resistor. This value changes with the amount of oxygen present in the air sample. Oxygen sensors usually

recommend a particular load resistor value or specify a range of acceptable values for the load resistor. Oxygen

sensors typically have a life of one to two years. The use of the micropower LMP2231 means minimal power

usage by the op amp and it enhances the battery life. Depending on other components present in the circuit

design, the battery could last for the entire life of the oxygen sensor. The precision specifications of the

LMP2231, such as its very low offset voltage, low TCVOS , low input bias current, low CMRR, and low PSRR are

other factors which make the LMP2231 a great choice for this application.

Figure 53. Precision Oxygen Sensor

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