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AD8022ARMZ 参数 Datasheet PDF下载

AD8022ARMZ图片预览
型号: AD8022ARMZ
PDF下载: 下载PDF文件 查看货源
内容描述: 双高速,低噪声运算放大器 [Dual High Speed, Low Noise Op Amp]
分类和应用: 运算放大器放大器电路光电二极管
文件页数/大小: 16 页 / 373 K
品牌: ADI [ ADI ]
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AD8022  
APPLICATIONS  
empty frequency bin. MTPR, sometimes referred to as the  
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.  
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.  
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.  
60  
50  
40  
30  
20  
10  
0
0
5
10  
15  
BITS/TONE  
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.  
In other words, a selected subband (or tone) remains open or  
void of intentional power (without a QAM signal) yielding an  
Rev. B | Page 13 of 16