ADM207E AD [Analog Devices], ADM207E Datasheet - Page 13

• Manufacturer Part Number: ADM207E
• Description: EMI/EMC Compliant, +-15 kV ESD Protected, RS-232 Line Drivers/Receivers
• Manufacturer: AD [Analog Devices]

REV. B
Conducted emissions are measured by monitoring the line
power supply. The equipment used consists of a LISN (Line
Impedance Stabilizing Network) which essentially presents a
fixed impedance at RF, and a spectrum analyzer. The spectrum
analyzer scans for emissions up to 30 MHz and a plot for the
ADM211E is shown in Figure 32.
OSCILLATOR
GND
INTERNAL
V
CC
80
70
60
50
40
30
20
10
0
Figure 30. Charge Pump Voltage Doubler
ø
ø
Figure 32. Conducted Emissions Plot
1
2
0.3
SWITCHING GLITCHES
S1
S2
Figure 31. Switching Glitches
0.6
LOG FREQUENCY – MHz
C1
1
3
S3
S4
6
C3
18
ADM206E/ADM207E/ADM208E/ADM211E/ADM213E
30
V
V+ = 2V
CC
LIMIT
CC
–13–
RADIATED EMISSIONS
Radiated emissions are measured at frequencies in excess of
30 MHz. RS-232 outputs designed for operation at high baud
rates while driving cables can radiate high frequency EM energy.
The reasons already discussed which cause conducted emissions
can also be responsible for radiated emissions. Fast RS-232
output transitions can radiate interference, especially when
lightly loaded and driving unshielded cables. Charge pump
devices are also prone to radiating noise due to the high fre-
quency oscillator and high voltages being switched by the charge
pump. The move towards smaller capacitors in order to con-
serve board space has resulted in higher frequency oscillators
being employed in the charge pump design. This has resulted in
higher levels of emission, both conducted and radiated.
The RS-232 outputs on the ADM2xxE products feature a con-
trolled slew rate in order to minimize the level of radiated emis-
sions, yet are fast enough to support data rates up to 230 kBaud.
Figure 34 shows a plot of radiated emissions vs. frequency. This
shows that the levels of emissions are well within specifications
without the need for any additional shielding or filtering compo-
nents. The ADM2xxE was operated at maximum baud rates
and configured as in a typical RS-232 interface.
Testing for radiated emissions was carried out in a shielded
anechoic chamber.
80
70
60
50
40
30
20
10
0
Figure 33. Radiated Emissions Test Setup
START 30.0 MHz
Figure 34. Radiated Emissions Plot
DUT
RADIATED NOISE
TURNTABLE
ADJUSTABLE
ANTENNA
STOP 200.0 MHz
TO
RECEIVER
LIMIT