Data Sheet AD8022
Rev. C | Page 13 of 16
5
APPLICATIONS
The low noise AD8022 dual xDSL receiver amplifier is
specifically designed for the dual differential receiver amplifier
function within xDSL transceiver hybrids, as well as other low
noise amplifier applications. The AD8022 can be used in
receiving modulated signals including discrete multitone
(DMT) on either end of the subscriber loop. Communication
systems designers can be challenged when designing an xDSL
modem transceiver hybrid capable of receiving the smallest
signals embedded in noise that inherently exists on twisted-pair
phone lines. Noise sources include near-end crosstalk (NEXT),
far-end crosstalk (FEXT), background, and impulse noise, all of
which are fed, to some degree, into the receiver front end. Based
on a Bellcore noise survey, the background noise level for
typical twisted-pair telephone loops is −140 dBm/√Hz or
31 nV/√Hz. It is therefore important to minimize the noise
added by the receiver amplifiers to preserve as much signal-to-
noise ratio (SNR) as possible. With careful transceiver hybrid
design, using the AD8022 dual, low noise, receiver amplifier to
maintain power density levels lower than −140 dBm/√Hz in
ADSL modems is easily achieved.
DMT MODULATION AND MULTITONE POWER
RATIO (MTPR)
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 is uniformly separated in frequency. (See Figure 24
to Figure 27 for MTPR results while the AD8022 receives DMT
driving 800 mV rms across a 500 Ω differential load.) A
uniquely encoded quadrature amplitude modulation (QAM)
signal occurs at the center frequency of each subband or tone.
Difficulties exist when decoding these subbands if a QAM
signal from one subband is corrupted by the QAM signal(s)
from other subbands, regardless of whether the corruption
comes from an adjacent subband or harmonics of other
subbands. Conventional methods of expressing the output
signal integrity of line receivers, such as spurious-free dynamic
range (SFDR), single tone harmonic distortion (THD), two-
tone intermodulation distortion (IMD), and third-order
intercept (IP3), become significantly less meaningful when
amplifiers are required to process DMT and other heavily
modulated waveforms. A typical xDSL downstream DMT signal
can contain as many as 256 carriers (subbands or tones) of
QAM signals. MTPR is the relative difference between the
measured power in a typical subband (at one tone or carrier) vs.
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
components. Measurements of MTPR are typically made at the
output of the receiver directly across the differential load. Other
components aside, the receiver function of an ADSL transceiver
hybrid is affected by the turns ratio of the selected transformers
within the hybrid design. Since a transformer reflects the
secondary voltage back to the primary side by the inverse of the
turns ratio, 1/N, increasing the turns ratio on the secondary side
reduces the voltage across the primary side inputs of the
differential receiver. Increasing the turns ratio of the
transformers can inadvertently cause a reduction of the SNR by
reducing the received signal strength.
CHANNEL CAPACITY AND SNR
The efficiency of an ADSL system in delivering the digital data
embedded in the DMT signals can be compromised when the
noise power of the transmission system increases. Figure 39
shows the relationship between SNR and the relative maximum
number of bits per tone or subband while maintaining a bit
error rate at 10
–7
errors per second.
BITS/TONE
SNR (dB)
60
0
10
20
30
40
50
01105
01053-039
Figure 39. ADSL DMT SNR vs. Bits/Tone
POWER SUPPLY AND DECOUPLING
The AD8022 should be powered with a good quality (that is,
low noise) dual supply of ±12 V for the best overall
performance. The AD8022 circuit also functions at voltages
lower than ±12 V. Careful attention must be paid to decoupling
the power supply pins. A pair of 10 μF capacitors located in
near proximity to the AD8022 is required to provide good
decoupling for lower frequency signals. In addition, 0.1 μF
decoupling capacitors should be located as close to each of the
power supply pins as is physically possible.
