ADM213E_15 Datasheet, PDF (14/20 Pages) Analog Devices – RS-232 Line Drivers/Receivers

ADM206E/ADM207E/ADM208E/ADM211E/ADM213E
Table 10. Test Severity Levels (IEC 1000-4-3)
Level
Field Strength (V/m)
1
1
2
3
3
10
EMISSIONS/INTERFERENCE
EN 55022, CISPR 22 defines the permitted limits of radiated
and conducted interference from information technology (IT)
equipment. The objective of the standard is to minimize the
level of emissions, both conducted and radiated.
For ease of measurement and analysis, conducted emissions are
assumed to predominate below 30 MHz, and radiated emissions
are assumed to predominate above 30 MHz.
CONDUCTED EMISSIONS
This is a measure of noise that is conducted onto the line power
supply. Switching transients from the charge pump that are 20 V
in magnitude and that contain significant energy can lead to
conducted emissions. Another source of conducted emissions is
the overlap in switch-on times in the charge pump voltage
converter. In the voltage doubler shown in Figure 31, if S2 has
not fully turned off before S4 turns on, a transient current glitch
occurs between VCC and GND that results in conducted emis-
sions. Therefore, it is important that the switches in the charge
pump guarantee break-before-make switching under all condi-
tions so instantaneous short-circuit conditions do not occur.
The ADM2xxE have been designed to minimize the switching
transients and ensure break-before-make switching, thereby
minimizing conducted emissions. This results in emission
levels well below specified limits. Other than the recom-
mended 0.1 μF capacitor, no additional filtering/decoupling
is required.
Conducted emissions are measured by monitoring the line
power supply. The equipment used consists of a line impedance
stabilizing network (LISN) that essentially presents a fixed
impedance at RF and a spectrum analyzer. The spectrum
analyzer scans for emissions up to 30 MHz. A plot for the
ADM211E is shown in Figure 33.
V_CC
GND
S1
S3
+
C1
S2
S4
+
C3
V+ = 2V_CC
V_CC
INTERNAL
OSCILLATOR
Figure 31. Charge Pump Voltage Doubler
ø1
ø2
SWITCHING GLITCHES
Figure 32. Switching Glitches
80
70
60
LIMIT
50
40
30
20
10
0
0.33 0.6 1
3
6 18
30
LOG FREQUENCY (MHz)
Figure 33. Conducted Emissions Plot
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 previously described causes of conducted emissions
can also cause radiated emissions. Fast RS-232 output tran-
sitions can radiate interference, especially when lightly loaded
and driving unshielded cables. Charge pump devices are also
prone to radiating noise due to the high frequency oscillator
and the high voltages being switched by the charge pump. The
move toward smaller capacitors in order to conserve board
space has resulted in higher frequency oscillators being em-
ployed in the charge pump design, resulting in higher levels of
conducted and radiated emissions.
The RS-232 outputs on the ADM2xxE products feature a con-
trolled slew rate in order to minimize the level of radiated
emissions, yet they are fast enough to support data rates of up to
230 kbps.
Rev. E | Page 14 of 20