LMV831MG National Semiconductor, LMV831MG Datasheet - Page 15

• Manufacturer Part Number: LMV831MG
• Description: OP AMP, 5V, EMI HARDENED, POWERWISE
• Manufacturer: National Semiconductor

Specifications of LMV831MG

• Op Amp Type: CMOS
• No. Of Amplifiers: 1
• Bandwidth: 3.3MHz
• Slew Rate: 2V/µs
• Supply Voltage Range: 2.7V To 5.5V
• Amplifier Case Style: SC-70
• No. Of Pins: 5
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The circuit diagram is shown in Figure 5. The PCB trace from
RF
the RF impedance of the cabling and the RF generator. On
the PCB a 50Ω termination is used. This 50Ω resistor is also
used to set the bias level of the IN+ pin to ground level. For
determining the EMIRR, two measurements are needed: one
is measuring the DC output level when the RF signal is off;
and the other is measuring the DC output level when the RF
signal is switched on. The difference of the two DC levels is
the output voltage shift as a result of the RF signal. As the op
amp is in the unity gain configuration, the input referred offset
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. The application is shown in Figure 7.
This application needs two op amps and therefore a dual op
amp is used. The op amp configured as a buffer and con-
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
dual supplies, place one capacitor between V
ground, and a second capacitor between ground and V
IN
FIGURE 5. Circuit for coupling the RF signal to IN+
to the IN+ pin should be a 50Ω stripline in order to match
FIGURE 7. Pressure Sensor Application
+
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and the board
+
and V
−
. For
−
.
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nected at the negative output of the pressure sensor prevents
the loading of the bridge by resistor R2. The buffer also pre-
vents the resistors of the sensor from affecting the gain of the
following gain stage. The op amps are placed in a single sup-
ply configuration.
The experiment is performed on two different dual op amps:
a typical standard op amp and the LMV832, EMI hardened
dual op amp. A cell phone is placed on a fixed position a cou-
ple of centimeters from the op amps in the sensor circuit.
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 LMV832, EMI hard-
ened op amp does not show any significant disturbances.
This means that the RF signal will not disturb the signal en-
tering the ADC when using the LMV832.
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 re-
maining EMI can be, almost, completely eliminated in the chip
by the EMI reducing features of the LMV831/LMV832/
LMV834.
FIGURE 6. Comparing EMI Robustness
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