Datasheet

TIME (0.5s/DIV)
V
OUT
(0.5V/DIV)
Typical Opamp
LMV832
+
-
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
LMV831, LMV832, LMV834
www.ti.com SNOSAZ6B –AUGUST 2008–REVISED MARCH 2013
signal level at the pin. The circuit diagram is shown in Figure 51. 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 51. 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 shown in Figure 53.
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 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 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 52.
Figure 52. Comparing EMI Robustness
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