LMP2234 Datasheet, PDF (18/20 Pages) National Semiconductor (TI) – Micropower, Precision, RRO Amplifier with CMOS Input

FIGURE 5. Strain Gauge Bridge Amplifier
20203468
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 deliv-
ered 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 oxy-
gen. 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 very small traces of oxy-
gen in ppm.
Figure 6 shows a typical circuit used to amplify the output
signal of an oxygen detector. The LMP2234 makes an excel-
lent choice for this application as it draws only 36 µA of current
and operates on supply voltages down to 1.8V. This applica-
tion 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 LMP2234 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 preci-
sion specifications of the LMP2234, such as its very low offset
voltage, low TCV_OS, low input bias current, low CMRR, and
low PSRR are other factors which make the LMP2234 a great
choice for this application.
FIGURE 6. Precision Oxygen Sensor
20203449
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