3222/3223-DB1 Silicon Laboratories Inc, 3222/3223-DB1 Datasheet - Page 33

• Manufacturer Part Number: 3222/3223-DB1
• Description: EVAL BOARD FOR IA3222/3223
• Manufacturer: Silicon Laboratories Inc
• Series: EZ DAA™r

Specifications of 3222/3223-DB1

• Main Purpose: Telecom, Data Acquisition Arrangement (DAA)
• Embedded: Yes, FPGA / CPLD
• Utilized Ic / Part: IA3222, IA3223
• Primary Attributes: -
• Secondary Attributes: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

6.12. Measuring Loop-Current Changes through the Received Audio Signal
The Line Side senses line-current information and encodes it for the System Side as a dc offset superimposed onto
the received audio data. Since modem DSP algorithms routinely remove low-frequency components from the
incoming data stream, dc offset is not a problem, but it needs to be taken into account in headroom calculations.
The current sensor has considerable dc offset, which needs to be calibrated to obtain good current-sensor
performance. This is achieved by adding a dc component to the transmitted data proportional to the received dc
offset using the following algorithm:
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6.13. 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 due to lightning, ESD, or incompatibility with digital PBX lines.
The failures are both metallic (differential) and longitudinal (common mode). Metallic failures are evidenced by
damage to components on the line side while longitudinal failures usually damage the drivers or receivers on either
side of the isolation barrier. Overdesigning for surge immunity can add as much as a dollar in costly surge
components compared to what is necessary to pass required regulatory testing or withstand field stresses. 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 classic example of inherent isolation is the standard telephone, which is not connected to the ac
mains. Answering machines and cordless telephones are completely insulated despite being powered by the
mains because 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 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.
6.14. Lightning Surges
Lightning rarely strikes the phone line directly. Lightning surges typically couple into the telephone line via several
different mechanisms. Lightning can strike 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. Although the twisted-
pair phone cable has a conductive sheath around it that is grounded periodically, its effectiveness is limited by its
own return inductance and resistance through the ground path. The power lines and the telephone cable form a
very low impedance pulse transformer that couples the lightning surge as a common-mode (longitudinal) transient
to the phone line. The net effect is that several kV of longitudinal transients can be put on the telephone line for any
lightning strike on the power line that runs above the same telephone lines.
Disable the current sensor by setting bit LP1 in the Line-Side LSB programming register. This cancels the dc
component due to the loop current itself and leaves the current sensor's offset component as dc offset in the
received data stream.
Add a small amount (20 to 50 mV) of dc offset to the outgoing data and note the amount of change in dc offset
in the incoming data. The ratio of incoming to outgoing dc offset changes is the dc-offset correction factor, for
which the sign must be retained.
Add a dc offset to all transmitted data equal to the received dc offset divided by the dc-offset correction factor,
based on the desired dc reference for the received signal. This is normally the same voltage as that of the
ACREF pin.
Enable the current sensor. The loop current can now be read as incoming-data dc offset from the dc reference
voltage. The sensitivity of the current sensor is approximately 1.25 mV of dc offset for every 1 mA of loop
current. Note that both the dc-offset correction factor and the gain change with the line-side termination
impedance setting.
Rev. 5.0
IA3222/23
33