ADM3311 AD [Analog Devices], ADM3311 Datasheet - Page 11

• Manufacturer Part Number: ADM3311
• Manufacturer: AD [Analog Devices]

EMISSIONS/INTERFERENCE
EN55022, CISPR22 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, which are
20 V in magnitude and contain significant energy, can lead to
conducted emissions. Other sources of conducted emissions can
be due to overlap in switch on times in the charge pump voltage
converter. In the voltage tripler shown in Figure 27, if S2 has
not fully turned off before S4 turns on, this results in a transient
current glitch between V
emissions. It is therefore important that the switches in the charge
pump guarantee break-before-make switching under all condi-
tions so that instantaneous short circuit conditions do not occur.
The ADM3311E has been designed to minimize the switching
transients and ensure break-before-make switching thereby
minimizing conducted emissions. This has resulted in the level
of emissions being well below the limits required by the specifi-
cation. No additional filtering/decoupling other than the recom-
mended 0.1 µF capacitor is required.
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
ADM3311E is shown in Figure 28.
GND
OSCILLATOR
V
CC
INTERNAL
Table V. Test Severity Levels (IEC1000-4-3)
S2
Level
1
2
3
S1
C1
+
CC
S4
S3
and GND which results in conducted
V
CC
S5
Field Strength
V/m
1
3
10
C2
+
S6
S7
C4
+
V+ = 3V
V
CC
CC
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 out-
put 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 frequency oscillator
and 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 employed in the
charge pump design. This has resulted in higher levels of emis-
sion, both conducted and radiated.
The RS-232 outputs on the ADM3311E products feature a
controlled slew rate in order to minimize the level of radiated emis-
sions, yet are fast enough to support data rates up to 230 kBaud.
90
80
70
60
50
40
30
20
10
100k
DUT
RADIATED NOISE
CONDUCTED QUASI-PEAK dB V
TURNTABLE
1
2
EN 55022 CLASS B
SWITCHING GLITCHES
FREQUENCY – Hz
1M
ADJUSTABLE
ANTENNA
ADM3311E
10M
TO
RECEIVER