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參數(shù)資料
| 型號： | AD768ARZ-REEL |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 6/20頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC DAC 16BIT 30MSPS 28-SOIC |
| 產(chǎn)品培訓(xùn)模塊： | Data Converter Fundamentals DAC Architectures |
| 標準包裝： | 1,000 |
| 設(shè)置時間： | 25ns |
| 位數(shù)： | 16 |
| 數(shù)據(jù)接口： | 并聯(lián) |
| 轉(zhuǎn)換器數(shù)目： | 1 |
| 電壓電源： | 雙 ± |
| 功率耗散（最大）： | 600mW |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 28-SOIC（0.295"，7.50mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 28-SOIC W |
| 包裝： | 帶卷 (TR) |
| 輸出數(shù)目和類型： | 2 電流，單極；2 電流，雙極 |
| 采樣率（每秒）： | 40M |
REV. B
–14–
AD768
AD768 IN MULTITONE TRANSMITTERS (FOR ADSL)
Communications applications frequently require aspects of
component performance that differ significantly from the
simple, single tone signals used in typical SNR and THD tests.
This is particularly true for spread-spectrum and frequency divi-
sion multiplexed (FDM) type signals, where information con-
tent is held in a number of small signal components spread
across the frequency band. In these applications, a combination
of wide dynamic range, good fine-scale linearity, and low inter-
modulation distortion is required. Unfortunately, a part’s full
scale SNR and THD performance may not be a reliable indica-
tor of how it will perform in these multitone applications.
One example of an FDM communications system is the DMT
(discrete multitone) ADSL (Asymmetrical Digital Subscriber
Line) standard currently being considered by ANSI. Figure 33
shows a block diagram of a transmitter function.
The digital bits are used to QAM modulate each of approxi-
mately 200 discrete tones. An inverse FFT turns this modu-
lated frequency domain information into 512 time points at a
2.2 MSPS sample rate. These time points are then put through
an FIR interpolation filter to upsample (in this case to 4.4 MSPS).
The bit stream is run through the AD768, which is followed by
a 4th order analog smoothing filter, then run to the line-driving
circuitry
TO
TRANSMITTER
BIT
STREAM
QAM
ENCODER
256
MODULATED
FREQUENCY
BINS
512 TIME
POINTS
@ 2.2MSPS
1024 TIME
POINTS
@ 4.4MSPS
4TH ORDER
SMOOTHING
FILTER
AD768
+BUFFER
2X
INTERPOLATOR
FIR
INVERSE
FFT
Figure 33. Typical DMT ADSL Transmit Chain
Figure 34a shows a frequency domain representation of a test
vector run through this system, while 34b shows the time do-
main representation. (Clearly the frequency domain picture is
more informative.) We wish to optimize the SINAD of each
4 kHz frequency band: this is a function of both noise
(wideband and quantization) and distortion (simple harmonic
and intermod).
OUTPUT
–
dB
FREQUENCY – Hz
0
–20
–40
–60
0
1.1M
–80
Figure 34a. Output Spectrum of ADSL Test Vector
TIME – 25s/DIV
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
OUTPUT
–
Volts
Figure 34b. Time Domain Output Signal of ADSL Test
Vector
Table I and II show the available SNR and THD at the output
of the filter vs. frequency bin for the ADSL application. The
AD768’s combination of 16-bit dynamic range and 14-bit lin-
earity provides excellent performance for the DMT signal. Its
fast input rate would support even faster rates of oversampling,
if one were interested in trading off digital filter complexity in
the interpolator for a simplified analog filter.
Table I. SNR vs. Frequency
Frequency
SNR
151 kHz
70.1 dB
349 kHz
69.7 dB
500 kHz
69.4 dB
1 MHz
69.8 dB
Table II. THD vs. Frequency
Frequency
THD
160 kHz
–68.9 dBc
418 kHz
–64.0 dBc
640 kHz
–64.3 dBc
893 kHz
–63.8 dBc
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