參數(shù)資料
型號(hào)： AD768AR
廠商： ANALOG DEVICES INC
元件分類： DAC
英文描述： 16-Bit, 30 MSPS D/A Converter
中文描述： PARALLEL, WORD INPUT LOADING, 0.025 us SETTLING TIME, 16-BIT DAC, PDSO28
封裝： 0.300 INCH, SOIC-28
文件頁(yè)數(shù)： 14/20頁(yè)
文件大?。?/td> 334K
代理商： AD768AR
REV. B
–14–
AD768
AD768 IN MULT IT ONE T RANSMIT T E RS (FOR ADSL)
Communications applications frequently require aspects of
component performance that differ significantly from the
simple, single tone signals used in typical SNR and T HD tests.
T his 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 T HD 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.
T he 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. T hese time points are then put through
an FIR interpolation filter to upsample (in this case to 4.4MSPS).
T he 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).
O
FREQUENCY – Hz
0
–20
–40
–60
0
1.1M
–80
Figure 34a. Output Spectrum of ADSL Test Vector
TIME – 25μs/DIV
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
O
Figure 34b. Time Domain Output Signal of ADSL Test
Vector
T able I and II show the available SNR and T HD at the output
of the filter vs. frequency bin for the ADSL application. T he
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.
T able I. SNR vs. Frequency
Frequency
SNR
151 kHz
349 kHz
500 kHz
1 MHz
70.1 dB
69.7 dB
69.4 dB
69.8 dB
T able II. T HD vs. Frequency
Frequency
T HD
160 kHz
418 kHz
640 kHz
893 kHz
–68.9 dBc
–64.0 dBc
–64.3 dBc
–63.8 dBc
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