ADM211EARU(1998) Просмотр технического описания (PDF) - Analog Devices

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ADM211EARU
(Rev.:1998)

EMI/EMC-Compliant, ±15 kV ESD­ Protected, RS-232 Line Drivers/Receivers

Analog Devices 

ADM206E/ADM207E/ADM208E/ADM211E/ADM213E

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.

S1

VCC

GND

C1

S2

S3

C3

S4

V+ = 2VCC

VCC

INTERNAL

OSCILLATOR

Figure 30. Charge Pump Voltage Doubler

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.

ø1

RADIATED NOISE

ø2

SWITCHING GLITCHES

DUT

TURNTABLE

ADJUSTABLE

ANTENNA

TO

RECEIVER

Figure 31. Switching Glitches

80

70

60

LIMIT

50

40

30

20

10

0

0.3 0.6 1

3

6 18

30

LOG FREQUENCY – MHz

Figure 32. Conducted Emissions Plot

Figure 33. Radiated Emissions Test Setup

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

LIMIT

30

20

10

0

START 30.0 MHz

STOP 200.0 MHz

Figure 34. Radiated Emissions Plot

REV. B

–13–

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