LMV831MGE NSC [National Semiconductor], LMV831MGE Datasheet - Page 16

• Manufacturer Part Number: LMV831MGE
• Description: 3.3 MHz Low Power CMOS, EMI Hardened Operational Amplifiers
• Manufacturer: NSC [National Semiconductor]

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PRESSURE SENSOR APPLICATION
The LMV831/LMV832/LMV834 can be used for pressure sen-
sor applications. Because of their low power the LMV831/
LMV832/LMV834 are ideal for portable applications, such as
blood pressure measurement devices, or portable barome-
ters. This example describes a universal pressure sensor that
can be used as a starting point for different types of sensors
and applications.
Pressure Sensor Characteristics
The pressure sensor used in this example functions as a
Wheatstone bridge. The value of the resistors in the bridge
change when pressure is applied to the sensor. This change
of the resistor values will result in a differential output voltage,
depending on the sensitivity of the sensor and the applied
pressure. The difference between the output at full scale
pressure and the output at zero pressure is defined as the
span of the pressure sensor. A typical value for the span is
100 mV. A typical value for the resistors in the bridge is
5 kΩ. Loading of the resistor bridge could result in incorrect
output voltages of the sensor. Therefore the selection of the
circuit configuration, which connects to the sensor, should
take into account a minimum loading of the sensor.
16
Pressure Sensor Example
The configuration shown in Figure 7 is simple, and is very
useful for the read out of pressure sensors. With two op amps
in this application, the dual LMV832 fits very well. The op amp
configured as a buffer and connected at the negative output
of the pressure sensor prevents the loading of the bridge by
resistor R2. The buffer also prevents the resistors of the sen-
sor from affecting the gain of the following gain stage. Given
the differential output voltage V
output signal of this op amp configuration, V
To align the pressure range with the full range of an ADC, the
power supply voltage and the span of the pressure sensor are
needed. For this example a power supply of 5V is used and
the span of the sensor is 100 mV. When a 100Ω resistor is
used for R2, and a 2.4 kΩ resistor is used for R1, the maxi-
mum voltage at the output is 4.95V and the minimum voltage
is 0.05V. This signal is covering almost the full input range of
the ADC. Further processing can take place in the micropro-
cessor following the ADC.
S
of the pressure sensor, the
OUT
, equals: