LMV862MMEVAL National Semiconductor, LMV862MMEVAL Datasheet - Page 15

• Manufacturer Part Number: LMV862MMEVAL
• Manufacturer: National Semiconductor

Specifications of LMV862MMEVAL

• Lead Free Status / Rohs Status: Supplier Unconfirmed

amp is connected in the unity gain configuration. Applying the
RF signal is straightforward as it can be connected directly to
the IN+ pin. As a result the RF signal path has a minimum of
components that might affect the RF signal level at the pin.
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 show in Figure 7.
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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This application needs two op amps and therefore a dual op
amp is used. The op amp configured as a buffer and con-
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 op amps: a typ-
ical standard op amp and the LMV862, EMI hardened dual op
amp. A cell phone is placed on a fixed position a couple 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 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 LMV862, 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 LMV862.
FIGURE 6. Comparing EMI Robustness
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