Datasheet

+
-
R
1
50:
RFin
C
1
22 pF
C
2
10 µF
100 pF
V
DD
V
SS
Out
+
C
3
100 pF
10 µF
+
C
4
C
5
LMV861, LMV862
SNOSAZ5C –FEBRUARY 2008–REVISED MARCH 2013 www.ti.com
Coupling an RF Signal to the IN+ Pin
Each of the op amp pins can be tested separately on EMIRR. In this section the measurements on the IN+ pin
(which, based on symmetry considerations, also apply to the IN- pin) are discussed. In Application Note AN-1698
the other pins of the op amp are treated as well. For testing the IN+ pin the op 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 48. The PCB trace from RF
IN
to the IN+ pin should be a 50Ω stripline in order to
match 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.
Figure 48. Circuit for coupling the RF signal to IN+
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 50.
This application needs two op amps and therefore a dual op amp is used. 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. The op
amps are placed in a single supply configuration.
The experiment is performed on two different op amps: a typical 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 49.
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