AD8022

Rev. B | 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

BITS/TONE

60

0

10

20

30

40

50

01

10

5

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.