ia3222 integration, ia3222 Datasheet - Page 35

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

IA3222/IA3223
SURGES, ISOLATION AND EMC
Among the three regulatory domains that DAAs must comply with (telecom, safety and EMC), safety and EMC tend to be highly
intertwined. Designing for regulatory approval can sometimes compromise field reliability of DAAs. Historically, the dominant cause
for field failures of modems or other DAA-based telephone products has been electrical overstress from the telephone line, typically
due to lightning, ESD, or incompatibility with digital PBX lines. The most common failures are both metallic (differential) and
longitudinal (common mode). Metallic failures are evidenced by damage to components on the line side while longitudinal isolation
failures usually damage the drivers or receivers on either side of the isolation barrier. Overdesigning for surge immunity is not
uncommon with DAAs. This can add as much as a dollar in costly surge components compared to what is necessary to pass required
regulatory testing or field stresses, which are often poorly understood. Even minimal surge and regulatory isolation components may
be the most expensive non-IC components in the DAA Bill of Materials. Moreover, contrary to expectation, more robust surge
components may actually make the DAA less robust overall.
For regulatory, functional and safety reasons, DAAs provide isolation and protection against excessive voltages and currents. The
telephone line system provides DC and AC common-mode ground at the current-source end of the line. At the user end or CPE
(Customer Premise End), the telephone device must be insulated either inherently or using a DAA.
The classic example of inherent isolation is the standard telephone, which is not connected to the AC mains. But commonly,
answering machines and cordless telephones also are completely insulated despite being powered by the mains. In these products,
two-prong transformer wall supplies provide the safety isolation. Products that have a third prong safety ground on the power plug
almost always use a DAA for the loop interface. If a product has a conductive chassis or has other electrical connections, it usually
will need a DAA to interface to the telephone line. Examples in this group are alarm systems, set-top boxes, fax machines, remote
meter readers, etc.
Functionally, longitudinal isolation at the CPE is the optimum solution for achieving very high common-mode noise rejection.
Repetitive longitudinal transients on the telephone line twisted pair may exceed 10V peak and continuous AC may exceed 70 V
RMS
(+40 dBm). Since the basic audible noise floor is around –75 dBm, this is over 110 dB of dynamic range. Only dielectric isolation
provides both very large common-mode range without overload and excellent common-mode rejection.
As in medical instrumentation systems, dielectric isolation, besides providing high common-mode immunity, also provides safety
isolation against electrical shock, overstress damage and fire. Generally, these latter issues are the concern of regulatory safety
standards. In many markets, the telephone line interface must satisfy these requirements strictly.
Consequently, DAAs provide a unique challenge in consumer products, needing to meet industrial isolation and robustness but at
consumer prices, unlike the industrial SLIC (Subscriber Line Interface Circuit) on the other end of the phone line. Traditionally, SLICs
are part of the total cost of the local telephone line that may have a capital value of $1000 that is amortized over a forty-year life. For
every industrial SLIC sold, there may be twenty consumer telephone devices sold.
Safety Isolation and Differential Surges
The sources of safety or damage causing electrical stress are lightning-induced transients, ESD (electrostatic discharge), AC power-
line crosses, AC power line transients, and incompatibility with digital PBX phone power. Proper DAA design can eliminate these
hazards.
Lightning-induced transients and ESD are very similar in behavior and the damage they cause. Generally, the best remedies for one
work well for the other. Lightning transients come down the telephone line from outside while ESD transients occur on CPE side.
Lightning rarely strikes the phone line directly, but more commonly couples into the telephone line via several different mechanisms.
One is that it strikes the high-voltage distribution lines on the same pole as the phone line. The strike may deliver a brief 1 kA pulse
down a hundred meters or more of power line before arcing to ground through the nearest power-distribution lightning arrestor.
Since the strike current runs parallel to the telephone lines, its very high dI/dt induces a large common-mode voltage in the parallel
phone cable. For example, if the lightning strike delivers 100 A/µs di/dt (1 kA over a 10 µs rise time) down a 100 µH power-line
inductance (say a 100 m power line at 1 µH/m), this generates a 10 kV inductive voltage down the power line. If the phone cable is
relatively close (e.g. 10 m) to the power line compared to the coupling length (e.g. 100 m), then a large percentage of this inductive
voltage will couple to the twisted pairs in the telephone cable below. Although the twisted-pair phone cable has a conductive sheath
around it that is grounded periodically and that acts as an eddy-current shield, its efficacy is limited by its own return inductance and
resistance through the ground path, which is often even further away than the inducing power lines. In other words, the power lines
and the telephone cable form a very low impedance pulse transformer that may couple to the telephone line up to 30 or 50 % of the
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