LMV852EVAL National Semiconductor, LMV852EVAL Datasheet - Page 16

• Manufacturer Part Number: LMV852EVAL
• Description: BOARD EVALUATION LMV852 8MHZ
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMV852EVAL

• Channels Per Ic: 2 - Dual
• Amplifier Type: General Purpose
• Output Type: Single-Ended, Rail-to-Rail
• Slew Rate: 4.5 V/µs
• Current - Output / Channel: 65mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 820µA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 5.5 V, ±1.35 V ~ 2.75 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMV852
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• -3db Bandwidth: -

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voltage shift corresponds one-to-one to the measured output
voltage shift.
Cell Phone Call
The effect of electromagnetic interference is demonstrated in
a setup where a cell phone interferes with a pressure sensor
application (Figure 7). This application needs two op amps
and therefore a dual op amp is used. The experiment is per-
formed on two different dual op amps: a typical standard op
amp and the LMV852, EMI hardened dual op amp. The op
amps are placed in a single supply configuration. The cell
phone is placed on a fixed position a couple of centimeters
from the op amps.
When the cell phone is called, the PCB and wiring connected
to the op amps receive the RF signal. Subsequently, the op
amps detect the RF voltages and currents that end up at their
pins. The resulting effect on the output of the second op amp
is shown in Figure 6.
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 LMV852, EMI hard-
ened op amp does not show any significant disturbances.
FIGURE 5. Circuit for Coupling the RF Signal to IN
FIGURE 6. Comparing EMI Robustness
20202168
20202167
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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
V
the board ground, and a second capacitor between ground
and V
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 LMV851/
LMV852/LMV854.
PRESSURE SENSOR APPLICATION
The LMV851/LMV852/LMV854 can be used for pressure sen-
sor applications. Because of their low power the LMV851/
LMV852/LMV854 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.
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 LMV852 fits very well.
The op amp configured as a buffer and connected at the neg-
ative 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
sensor, the output signal of this op amp configuration, V
equals:
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.
−
. For dual supplies, place one capacitor between V
−
. Even with the LMV851/LMV852/LMV854 inherent
S
of the pressure
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OUT
and
and
,