AD8022(1999) 查看數據表（PDF） - Analog Devices

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AD8022
(Rev.:1999)

Dual High-Speed Low-Noise Op Amps

Analog Devices 

AD8022

process DMT and other heavily modulated waveforms. A typical

xDSL downstream DMT signal may contain as many as 256

carriers (subbands or tones) of QAM signals. MTPR is the rela-

tive 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 dif-

ference 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 will be 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 pri-

mary side inputs of the differential receiver. Increasing the turns

ratio of the transformers may 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. The graph

below shows the relationship between SNR and the relative maxi-

mum number of bits per tone or subband while maintaining a bit

error rate at 1E-7 errors per second.

60.00

50.00

40.00

30.00

20.00

10.00

0.00

0

5

10

15

BITS/TONE

Figure 36. ADSL DMT SNR vs. Bits/Tone

Generating DMT

At this time, DMT modulated waveforms are not typically menu

selectable items contained within arbitrary waveform generators.

AWGs that are available today may not deliver DMT signals

sufficient in performance with regard to MTPR due to limita-

tions in the D/A converters and output amplifiers used by AWG

manufacturers. Similar to evaluating single tone distortion perfor-

mance of an amplifier, MTPR evaluation requires a DMT signal

generator capable of delivering MTPR performance better than

that of the driver under evaluation. Generating DMT signals can

be accomplished using a Tektronics AWG 2021 equipped with

Opt 4, (12-bit/24-bit, TTL Digital Data Out), digitally coupled

to Analog Devices’ AD9754, a 14-bit TxDAC, buffered by an

AD8002 amplifier configured as a differential driver. See Figure

37 for schematics of a circuit used to generate DMT signals

that can achieve down to –80 dBc of MTPR performance,

sufficient for use in evaluating xDSL receivers. WFM files are

needed to produce the necessary digital data required to drive

the TxDAC from the optional TTL Digital Data output of

the TEK AWG2021. Copies of .WFM files for upstream and

downstream DMT waveforms with a peak-to-average ratio (crest

factor) of ~5.3 can be obtained through the Analog Devices

web site. http://products.analog.com/products/info.asp?

product=AD8022

Upstream data is contained in the ...24.wfm files and downstream

data in the ...128.wfm files. These DMT modulated signals are

used to evaluate xDSL products for Multitone Power Ratio or

MTPR performance. The data files are used in pairs (adslu24.wfm

and adsll24.wfm go together, etc.) and are loaded into Tektronics

AWG2021 arbitrary waveform generator. The adslu24.wfm is

loaded via the TEK AWG2021 floppy drive into Channel 1

while the adsll24.wfm is simultaneously loaded into Channel

2. The number in the file name, prefixed with ‘u,’ goes into

CH1 or upper channel and the ‘l’ goes into CH2 or the lower

channel. Twelve bits from channel CH1 are combined with two

bits from CH2 to achieve 14-bit digital data at the digital out-

puts of the TEK 2021. The resulting waveforms produced at the

AD9754-EB outputs are then buffered and amplified by the

AD8002 differential driver to achieve 14-bit performance from

this DMT signal source.

Power Supply and Decoupling

The AD8022 should be powered with a good quality (i.e., low

noise) dual supply of ±12 V for the best overall performance.

The AD8022 circuit will also function 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 physi-

cally possible.

REV. 0

–11–

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