LPV521MGX Datasheet, PDF (20/29 Pages) Texas Instruments – LPV521 Nanopower, 1.8V, RRIO, CMOS Input, Operational Amplifier

LPV521
SNOSB14C – AUGUST 2009 – REVISED FEBRUARY 2013
I_CHARGE
R_SENSE
V⁺
LOAD
R₂
24.9 k:
10:
R₁
24.9 k:
V⁺
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R_SENSE X R₃
V_OUT =
R₁
X I_CHARGE
R₃
10 M:
Q1
2N2907
V_OUT
Figure 64. High Side Current Sensing
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 65 shows a typical circuit used to amplify the output of an oxygen detector. 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. 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 nanopower LPV521 means minimal power usage
by the op amp and it enhances the battery life. With the components shown in Figure 65 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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