AD9767AST datasheet(16/27 Pages) AD

16 / 27 page

REV. B

AD9767

–16–

Proper grounding and decoupling should be a primary objective

in any high speed, high resolution system. The AD9767 features

separate analog and digital supply and ground pins to optimize

the management of analog and digital ground currents in a

system. In general, AVDD, the analog supply, should be de-

coupled to ACOM, the analog common, as close to the chip as

physically possible. Similarly, DVDD, the digital supply, should

be decoupled to DCOM as close to the chip as physically possible.

For those applications that require a single +5 V or +3 V supply

for both the analog and digital supplies, a clean analog supply

may be generated using the circuit shown in Figure 39. The

circuit consists of a differential LC filter with separate power

supply and return lines. Lower noise can be attained by using

low ESR type electrolytic and tantalum capacitors.

100 F

10–22 F

0.1 F

TTL/CMOS

LOGIC

CIRCUITS

+5V

POWER SUPPLY

FERRITE

BEADS

AVDD

ACOM

ELECTROLYTIC

TANTALUM

CERAMIC

Figure 39. Differential LC Filter for Single +5 V and +3 V

Applications

APPLICATION

VDSL Applications Using the AD9767

Very High Frequency Digital Subscriber Line (VDSL) technol-

ogy is growing rapidly in applications requiring data transfer

over relatively short distances. By using QAM modulation and

transmitting the data in Discrete Multiple Tones (DMT), high

data rates can be achieved.

As with other multitone applications, each VDSL tone is ca-

pable of transmitting a given number of bits, depending on the

signal-to-noise ratio (SNR) in a narrow band around that tone.

For a typical VDSL application, the tones are evenly spaced

over the range of several kHz to 10 MHz. At the high frequency

end of this range, performance is generally limited by cable

characteristics and environmental factors, such as external inter-

ferers. Performance at the lower frequencies is much more de-

pendent on the performance of the components in the signal

chain. In addition to in-band noise, intermodulation from other

tones can also potentially interfere with the data recovery for a

given tone. The two graphs in Figure 40 represent a 500 tone

missing bin test vector, with frequencies evenly spaced from

400 Hz to 10 MHz. This test is very commonly done to deter-

mine if distortion will limit the number of bits which can be

transmitted in a tone. The test vector has a series of missing

tones around 750 kHz, which is represented in Figure 40a, and

a series of missing tones around 5 MHz, which is represented in

Figure 40b. In both cases, the spurious free dynamic range

(SFDR) between the transmitted tones and the empty bins is

greater than 60 dB.

FREQUENCY – MHz

0.665

–90

–80

–70

–60

–50

–40

–30

–20

–100

–120

–110

0.685 0.705 0.725 0.745 0.765 0.785 0.805 0.825

Figure 40a. Notch in Missing Bin at 750 kHz Is Down

>60 dB (Peak Amplitude = 0 dBm)

FREQUENCY – MHz

4.85

4.90

4.95

5.00

5.05

5.10

5.15

–80

–60

–40

–20

–100

–120

Figure 40b. Notch in Missing Bin at 5 MHz Is Down

>60 dB (Peak Amplitude = 0 dBm)

CDMA

Carrier Division Multiple Access, or CDMA, is an air transmit/

receive scheme where the signal in the transmit path is modu-

lated with a pseudorandom digital code (sometimes referred to

as the spreading code). The effect of this is to spread the trans-

mitted signal across a wide spectrum. Similar to a DMT wave-

form, a CDMA waveform containing multiple subscribers can

be characterized as having a high peak to average ratio (i.e.,

crest factor), thus demanding highly linear components in the

transmit signal path. The bandwidth of the spectrum is defined

by the CDMA standard being used, and in operation is imple-

mented by using a spreading code with particular characteristics.

Distortion in the transmit path can lead to power being trans-

mitted out of the defined band. The ratio of power transmitted

in-band to out-of-band is often referred to as Adjacent Channel

Power (ACP). This is a regulatory issue due to the possibility of

interference with other signals being transmitted by air. Regula-

tory bodies define a spectral mask outside of the transmit band,

and the ACP must fall under this mask. If distortion in the

transmit path causes the ACP to be above the spectral mask,

then filtering, or different component selection, is needed to

meet the mask requirements.

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