ia3222 integration, ia3222 Datasheet - Page 39

• Manufacturer Part Number: ia3222
• Description: Ia3222/ia3223 Ez Daa? Chipset With Analog Interface
• Manufacturer: integration

IA3222/IA3223
EMC
Another key advantage of the IA3222/3223 chipset is that many applications do not require the usual telephone-line EMC
suppression components. The 2 pF total value of the isolation capacitors presents significant impedance at VHF and UHF radiating
frequencies. At 300 MHz, 2 pF has a reactive impedance of 265 ohms— comparable to that of ferrite beads. This coupling
impedance is significantly higher than other capacitor DAAs where the isolation impedances can be ten times lower or even less.
Compared to these other DAAs, the chipset will couple better than 20 dB less RF to the phone line if no other RF suppression is used
and will be comparable to the other capacitor-coupled DAAs where ferrite beads are used.
If higher levels of RF suppression are required, adding EMI shunt capacitors between each side of the line to the chassis will be more
effective than adding ferrite beads. To minimize the need for EMI shunt capacitors, the line side area (antenna) should be minimized
and placed as close as possible to the phone line connector. Extending the chassis ground on each of line side (while still
maintaining minimum isolation creepage) will act as an RF shield. If board area is available, the EMI line capacitors can be fabricated
like the PCB isolation capacitors by using the upper and lower PCB layers. Matched 10 pF capacitors on each side of the line to the
chassis would attenuate line RF by about 20 db.
RF Susceptibility
For DAAs, large common-mode RF signals below 2 MHz can be a serious source of interference. In particular, AM radio-band
transmitters in the 500 kHz to 1.6 MHz range are common in urban areas. Although the field strength for AM radio is typically no
more than other urban signals (TV, FM, two-way radio, cell phones, etc.), the twisted-pair telephone line from the pole plus the
unshielded telephone line in the building makes an excellent long-wave antenna since neither of these are shielded like the multi-
pair cable on the pole. Because of the long wavelengths in this band (150 to 600 m), a quarter-wave long wire antenna has a large
RF capture area, providing up to a hundred times higher common-mode signal levels than the field strength per meter.
Consequently, it is not unusual to see many Volts of common-mode RF signal on the telephone line. This can easily be seen by
putting an oscilloscope probe on tip or ring. To prevent interference from these AM-band signals, some DAAs require both expensive
series inductors in the range of several µH and high-voltage shunt capacitors.
Without the need for these costly RF suppression components, the IA3222/3223 chipset achieves very high common-mode RF
immunity. This results from the combination of very low isolation capacitance and internal filtering. Typically, RF immunity of the
IA3222/3223 will be sufficient and the DAA will perform quite well without any special RF suppression components. On the other
hand, if immunity is desired to the level specified in EN 55024 or even to Brazil’s more demanding Anexo a Resolução No 237, a
pair of 470 pF high-voltage capacitors between Tip/Ring and the chassis ground is normally sufficient. In general, filtering
components work both ways: any RF solution that works well for radiated signals will work well for susceptibility in the same
frequency range.
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