AD8391ARZ Analog Devices Inc, AD8391ARZ Datasheet - Page 13

• Manufacturer Part Number: AD8391ARZ
• Description: IC LINE DRVR XDSL PWR DWN 8SOIC
• Manufacturer: Analog Devices Inc
• Type: Driverr

Specifications of AD8391ARZ

• Number Of Drivers/receivers: 2/0
• Protocol: xDSL
• Voltage - Supply: 3 V ~ 12 V
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• For Use With: AD8391AR-EVAL - BOARD EVAL FOR AD8391
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Receive Channel Considerations
A transformer used at the output of the differential line driver to
step up the differential output voltage to the line has the inverse
effect on signals received from the line. A voltage reduction or
attenuation equal to the inverse of the turns ratio is realized in the
receive channel of a typical bridge hybrid. The turns ratio of the
transformer may also be dictated by the ability of the receive
circuitry to resolve low level signals in the noisy twisted pair tele-
phone plant. While higher turns ratio transformers boost transmit
signals to the appropriate level, they also effectively reduce the
received signal-to-noise ratio due to the reduction in the
received signal strength. Using a transformer with as low a turns
ratio as possible will limit degradation of the received signal.
The AD8022, a dual amplifier with typical RTI voltage noise of
only 2.5 nV/ Hz and a low supply current of 4 mA/amplifier, is
recommended for the receive channel. If power-down is required
for the receive amplifier, two AD8021 low noise amplifiers can
be used instead.
DMT Modulation, Multitone Power Ratio (MTPR), and
Out-of-Band SFDR
ADSL systems rely on discrete multitone (DMT) modulation to
carry digital data over phone lines. DMT modulation appears in
the frequency domain as power contained in several individual
frequency subbands, sometimes referred to as tones or bins,
each of which are uniformly separated in frequency. A uniquely
encoded quadrature amplitude modulation (QAM) like signal
occurs at the center frequency of each subband or tone. See
Figure 7 for an example of a DMT waveform in the frequency
domain and Figure 8 for a time domain waveform. Difficulties
will exist when decoding these subbands if a QAM signal from
one subband is corrupted by the QAM signal(s) from other
subband regardless of whether the corruption comes from an
adjacent subband or harmonics of other subbands.
REV. A
Figure 7. DMT Waveform in the Frequency Domain
–80
–20
–40
–60
20
0
0
50
FREQUENCY – kHz
100
150
–13–
Conventional methods of expressing the output signal integrity of
line drivers, such as single-tone harmonic distortion or THD,
two-tone intermodulation distortion (IMD) and third-order
intercept (IP3) become significantly less meaningful when ampli-
fiers are required to process DMT and other heavily modulated
waveforms. A typical ADSL upstream DMT signal can contain
as many as 27 carriers (subbands or tones) of QAM signals.
Multitone power ratio (MTPR) is the relative difference between
the measured power in a typical subband (at one tone or carrier)
versus the power at another subband specifically selected to
contain no QAM data. In other words, a selected subband (or
tone) remains open or void of intentional power (without a QAM
signal) yielding an empty frequency bin. MTPR, sometimes
referred to as the empty bin test, is typically expressed in dBc,
similar to expressing the relative difference between single-tone
fundamentals and second or third harmonic distortion compo-
nents. Measurements of MTPR are typically made on the line
side or secondary side of the transformer.
TPC 21 and TPC 24 depict MTPR and SFDR versus trans-
former turns respectively for a variety of line power ranging from
12 dBm to 14 dBm. As the turns ratio increases, the driver hybrid
can deliver more undistorted power to the load due to the high
output current capability of the AD8391. Significant degradation
of MTPR will occur if the output transistors of the driver saturate,
causing clipping at the DMT voltage peaks. Driving DMT signals
to such extremes not only compromises in-band MTPR but will
also produce spurs that exist outside of the frequency spectrum
containing the transmitted signal. Out-of-band spurious-free
dynamic range (SFDR) can be defined as the relative difference in
amplitude between these spurs and a tone in one of the upstream
bins. Compromising out-of-band SFDR is the equivalent to
increasing near-end crosstalk (NEXT). Regardless of terminology,
maintaining high out-of-band SFDR while reducing NEXT will
improve the overall performance of the modems connected at either
end of the twisted pair.
–2
–3
–1
–0.25
4
3
2
1
0
Figure 8. DMT Signal in the Time Domain
–0.20 –1.50 –1.00 –0.05
TIME – ms
0
0.05
1.00
AD8391
1.50
0.20