LMV831_13 Datasheet, PDF (18/25 Pages) Texas Instruments – LMV831 Single/ LMV832 Dual/ LMV834 Quad 3.3 MHz Low Power CMOS, EMI Hardened

LMV831, LMV832, LMV834
SNOSAZ6A – AUGUST 2008 – REVISED OCTOBER 2008
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The difference between the two types of dual op amps is clearly visible. The typical standard dual op amp has an
output shift (disturbed signal) larger than 1V as a result of the RF signal transmitted by the cell phone. The
LMV832, EMI hardened op amp does not show any significant disturbances. This means that the RF signal will
not disturb the signal entering the ADC when using the LMV832.
V_DD
PRESSURE
SENSOR
+
-
-
LMV832
+
R2
100 :
R1
2.4 k:
V_DD
-
LMV832
+
V_OUT
ADC
Figure 53. Pressure Sensor Application
DECOUPLING AND LAYOUT
Care must be given when creating a board layout for the op amp. For decoupling the supply lines it is suggested
that 10 nF capacitors be placed as close as possible to the op amp. For single supply, place a capacitor between
V⁺ and V^−. For dual supplies, place one capacitor between V⁺ and the board ground, and a second capacitor
between ground and V^−. Even with the LMV831/LMV832/LMV834 inherent hardening against EMI, it is still
recommended to keep the input traces short and as far as possible from RF sources. Then the RF signals
entering the chip are as low as possible, and the remaining EMI can be, almost, completely eliminated in the chip
by the EMI reducing features of the LMV831/LMV832/LMV834.
PRESSURE SENSOR APPLICATION
The LMV831/LMV832/LMV834 can be used for pressure sensor applications. Because of their low power the
LMV831/LMV832/LMV834 are ideal for portable applications, such as blood pressure measurement devices, or
portable barometers. 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.
Pressure Sensor Example
The configuration shown in Figure 53 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 sensor from affecting the gain of the following gain stage. Given the differential
output voltage V_S of the pressure sensor, the output signal of this op amp configuration, V_OUT, equals:
V_OUT =
V_DD
2
-
V_S
2
¨¨©§^{1+ 2×} _RR21¸¸¹·
(2)
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 maximum 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 microprocessor following the ADC.
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