AD8016 datasheet(12/20 Pages) AD

12 / 20 page

REV. B

AD8016

–12–

BIAS pin (IBIAS) and the supply current (IQ). A typical shut-

down IQ is less than 1 mA total. Alternatively, an external pull-

down resistor to ground or a current sink attached to the BIAS

pin can be used to set IQ to lower levels (see Figure 39). The

BIAS pin may be used in combination with the PWDN1 and

PWDN0 pins; however, diminished MTPR performance may

result when IQ is lowered too much. Current pulled away from

the BIAS pin shunts away a portion of the internal bias current.

Setting PWDN1 or PWDN0 to Logic 0 also shunts away a

portion of the internal bias current. The reduction of quiescent

bias levels due to the use of PWDN1 and PWDN0 is consistent

with the percentages established in Table II. When PWDN0 alone

is set to Logic 0, and no other means of reducing the internal

bias currents is used, full-rate ADSL signals may be driven while

maintaining reasonable levels of MTPR.

50k

BIAS

3.3V LOGIC

2N3904

*R1 = 47k

FOR

12VS OR +12VS,

R1 = 22k

FOR

6VS.

Figure 38. Logic Drive of BIAS Pin for

Complete Amplifier Shutdown

THERMAL SHUTDOWN

The AD8016ARB and AD8016ARP have been designed to

incorporate shutdown protection against accidental thermal

overload. In the event of thermal overload, the AD8016 was

designed to shut down at a junction temperature of 165

°C and

return to normal operation at a junction temperature 140

°C.

The AD8016 continues to operate, cycling on and off, as long as

the thermal overload condition remains. The frequency of the

protection cycle depends on the ambient environment, severity

of the thermal overload condition, the power being dissipated,

and the thermal mass of the PCB beneath the AD8016. When

the AD8016 begins to cycle due to thermal stress, the internal

shutdown circuitry draws current out of the node connected in

common with the BIAS pin, while the voltage at the BIAS pin

goes to the negative rail. When the junction temperature returns

to 140

°C, current is no longer drawn from this node, and the

BIAS pin voltage returns to the positive rail. Under these cir-

cumstances, the BIAS pin can be used to trip an alarm indicat-

ing the presence of a thermal overload condition.

Figure 39 also shows three circuits for converting this signal to a

standard logic level.

VCC

BIAS

10k

ALARM

1/4 HCF 40109B

SGS–THOMSON

10k

100k

BIAS

ALARM

MIN

350

VEE

10k

200 A

VCC

BIAS

PWDN1

BIAS

0 A–200 A

V = VCC – 0.2V

SHUT-

DOWN

AD8016

PWDN0

Figure 39. Shutdown and Alarm Circuit

APPLICATIONS

The AD8016ARP and AD8016ARB dual xDSL line driver

amplifiers are the most efficient xDSL line drivers available on

the market today. The AD8016 may be applied in driving modu-

lated signals including discrete multitone (DMT) in either

direction; upstream from CPE to the CO and downstream from

CO to CPE. The most significant thermal management chal-

lenge lies in driving downstream information from CO sites to

the CPE. Driving xDSL information downstream suggests the

need to locate many xDSL modems in a single CO site. The

implication is that several modems will be placed onto a single

printed circuit board residing in a card cage located in a variety

of ambient conditions. Environmental conditioners such as fans

or air conditioning may or may not be available, depending on

the density of modems and the facilities contained at the CO site.

To achieve long-term reliability and consistent modem perfor-

mance, designers of CO solutions must consider the wide array

of ambient conditions that exist within various CO sites.

MULTITONE POWER RATIO OR MTPR

ADSL systems rely on discrete multitone 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 1 for

an example of downstream DMT signals used in evaluating

MTPR performance.) A uniquely encoded, quadrature amplitude

modulation (QAM) signal occurs at the center frequency of

each subband or tone. Difficulties arise 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 drivers, such as spurious-free

dynamic range (SFDR), single-tone harmonic distortion or

THD, two-tone intermodulation distortion (IMD), and third-

order intercept (IP3) become significantly less meaningful when

amplifiers are required to drive 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 relative difference between the mea-

sured 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, some-

times referred to as the empty bin test, is typically expressed

in dBc, similar to expressing the relative difference between

single-tone fundamentals and second or third harmonic dis-

tortion components.

See Figure 1 for a sample of the ADSL downstream spectrum

showing MTPR results while driving 20.4 dBm of power onto a

100

Ω line. Measurements of MTPR are typically made at the

output (line side) of ADSL hybrid circuits. (See Figure 46a for

an example of Analog Devices’ hybrid schematic.) MTPR can

be affected by the components contained in the hybrid circuit,

including the quality of the capacitor dielectrics, voltage ratings,

and the turns ratio of the selected transformers. Other compo-

nents aside, an ADSL driver hybrid containing the AD8016 can be

optimized for the best MTPR performance by selecting the turns

ratio of the transformers. The voltage and current demands from

the differential driver changes, depending on the transformer

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