AD8016ARB datasheet(12/20 Pages) AD

12 / 20 page

REV. A

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 will shunt away a portion of the internal bias cur-

rent. 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 ARP 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 will

continue 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 ther-

mal overload condition, the power being dissipated and the ther-

mal 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 circumstances,

the BIAS pin can be used to trip an alarm indicating the pres-

ence of a thermal overload condition.

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

standard logic level.

VCC

+5V

BIAS

10k

ALARM

1/4 HCF 40109B

SGS - THOMSON

10k

+5V

100k

BIAS

ALARM

MIN

350

VEE

10k

200 A

VCC

BIAS

PWDN1

BIAS

0–200 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 to

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

lated signals including Discrete Multitone (DMT) in either

direction; upstream from Customer Premise Equipment (CPE)

to the Central Office (CO) and downstream from CO to CPE.

The most significant thermal management challenge 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 cir-

cuit board residing in a card cage located in a variety of ambient

conditions. Environmental conditioners such as fans or air con-

ditioning 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 performance, designers

of CO solutions must consider the wide array of ambient condi-

tions that exist within various CO sites.

MULTITONE POWER RATIO OR MTPR

ADSL systems rely on Discrete Multitone (or DMT) 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

example of downstream DMT signals used in evaluating MTPR

performance.) A uniquely encoded, Quadrature Amplitude Modu-

lation (QAM) signal occurs at the center frequency of each

subband or tone. Difficulties will exist 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 3rd 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. Multitone Power

Ratio (MTPR) is the relative 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 difference between single-tone fundamentals and 2nd or

3rd harmonic distortion 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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