LPV521_15 Datasheet, PDF (23/33 Pages) Texas Instruments – LPV521 NanoPower, 1.8-V, RRIO, CMOS Input, Operational Amplifier

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Typical Applications (continued)
8.2.2 Portable Gas Detection Sensor
LPV521
SNOSB14D – AUGUST 2009 – REVISED DECEMBER 2014
1 M:
R_L
100 M:
V⁺
-
+
V^-
V_OUT
OXYGEN SENSOR
Figure 66. Precision Oxygen Sensor
8.2.2.1 Design Requirements
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. The LPV521 makes an excellent choice for this application
as it only draws 345 nA of current and operates on supply voltages down to 1.6V. 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. This
application detects oxygen in air. 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.
8.2.2.2 Detailed Design Procedure
Figure 66 shows a typical circuit used to amplify the output of an oxygen detector. 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. The use of the nanopower LPV521 means minimal power usage by the op amp and it
enhances the battery life. With the components shown in Figure 66 the circuit can consume less than 0.5 µA of
current ensuring that even batteries used in compact portable electronics, with low mAh charge ratings, could
last beyond the life of the oxygen sensor. The precision specifications of the LPV521, such as its very low offset
voltage, low TCV_OS , low input bias current, high CMRR, and high PSRR are other factors which make the
LPV521 a great choice for this application.
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