AD8016 datasheet(13/20 Pages) AD

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REV. B

AD8016

–13–

turns ratio. The point on the curve indicating maximum dynamic

headroom is achieved when the differential driver delivers both

the maximum voltage and current while maintaining the lowest

possible distortion. Below this point, the driver has reserve

current-driving capability and experiences voltage clipping.

Above this point, the amplifier runs out of current drive capabil-

ity before the maximum voltage drive capability is reached.

Since a transformer reflects the secondary load impedance back

to the primary side by the square of the turns ratio, varying the

turns ratio changes the load across the differential driver. In the

transformer configuration of Figure 46a and 46b, the turns ratio

of the selected transformer is effectively doubled due to the

parallel wiring of the transformer primaries within this ADSL

driver hybrid. The following equation may be used to calculate

the load impedance across the output of the differential driver,

reflected by the transformers, from the line side of the xDSL

driver hybrid. Z' is the primary side impedance as seen by the

differential driver; Z2 is the line impedance and N is the trans-

former turns ratio.

≡

()

Figure 40 shows the dynamic headroom in each subband of a

downstream DMT waveform versus turns ratio running at 100%

and 60% of the quiescent power while maintaining –65 dBc of

MTPR at VS =

±12 V.

1.0

1.2

1.4

2.0

–1

1.6

1.8

–2

1.1

1.3

1.5

1.7

1.9

DOWNSTREAM TURNS RATIO

VS = 12V

PWDN1, PWDN0 = (1,1)

VS = 11.4V

PWDN1, PWDN0 = (1,1)

VS = 11.4V

PWDN1, PWDN0 = (1,0)

VS = 12V

PWDN1, PWDN0 = (1,0)

Figure 40. Dynamic Headroom vs. XFMR Turns Ratio,

VS =

±12 V

Once an optimum turns ratio is determined, the amplifier has an

MTPR performance for each setting of the power-down pins.

The table below demonstrates the effects of reducing the total

power dissipated by using the PWDN pins on MTPR perfor-

mance when driving 20.4 dBm downstream onto the line with a

transformer turns ratio of 1:1.4.

Table III. Dynamic Power Dissipation for

Downstream Transmission

PWDN1

PWDN0

PD (W)

MTPR

1.454

–78 dBc

1.262

–75.3 dBc

1.142

–57.2 dBc

0.120

N/A

*This mode is quiescent power dissipation.

GENERATING DMT

At this time, DMT modulated waveforms are not typically menu-

selectable items contained within arbitrary waveform generators.

Even using AWG software to generate DMT signals, AWGs that

are available today may not deliver DMT signals sufficient in

performance with regard to MTPR due to limitations in the D/A

converters and output drivers used by AWG manufacturers.

Similar to evaluating single-tone distortion performance of an

amplifier, MTPR evaluation requires a DMT signal generator

capable of delivering MTPR performance better than that of the

driver under evaluation. Generating DMT signals can be accom-

plished using a Tektronics AWG 2021 equipped with opt 4,

(12/24-Bit, TTL digital data out), digitally coupled to Analog

Devices AD9754, a 14-bit TxDAC

®, buffered by an AD8002

amplifier configured as a differential driver. See Figure 45 for

schematics of a circuit used to generate DMT signals that can

achieve down to –80 dBc of MTPR performance, sufficient for

use in evaluating xDSL drivers. Note that the DMT waveforms

available with the AD8016ARP-EVAL and AD8016ARB-EVAL

boards or similar WFM files are needed to produce the neces-

sary digital data required to drive the TxDAC from the optional

TTL digital data output of the TEK AWG2021. Copies of these

WFM files can be obtained through the Analog Devices website,

at www.analog.com.

EVALUATION BOARDS

The AD8016ARP-EVAL, AD8016ARB-EVAL, AD8016ARE-

EVAL boards available through Analog Devices provide a platform

for evaluating the AD8016 in an ADSL differential line driver

circuit. The board is laid out to accommodate Analog Devices’

two transformer line driver hybrid circuits (see Figures 46a

and 46b) including line matching network, an RJ11 jack for

interfacing to line simulators, transformer coupled input for

single-to-differential input conversion, and accommodations for

the receiver function. Schematics and layout information are

available for both versions of the evaluation board. Also included

in the package are WFM files for use in generating 14-bit

DMT waveforms. Upstream data is contained in the ...24.wfm

files and downstream data in the ...128.wfm files.

These DMT modulated signals are used to evaluate xDSL

products for multitone power ratio or MTPR performance.

The data files are used in pairs (adslu24.wfm and adsll24.wfm

go together, etc.) and are loaded into a TEK AWG2021 arbi-

trary waveform generator. The adslu24.wfm is loaded via the

TEK AWG2021 floppy drive into Channel 1, while the

adsll24.wfm is simultaneously loaded into Channel 2. The num-

ber in the file name, prefixed with “u,” goes into CH1 or upper

channel and the “l” goes into CH2 or the lower channel. 12 bits

from CH1 are combined with 2 bits from CH2 to achieve 14-bit

digital data at the digital outputs of the TEK AWG2021. The

resulting waveforms produced at the AD9754-EB outputs are then

buffered and amplified by the AD8002 differential driver to achieve

14-bit performance from this DMT signal source.

POWER DISSIPATION

In order to properly size the heat sinking area for the user’s

application, it is important to consider the total power dissipa-

tion of the AD8016. The dc power dissipation for VIN = 0 is IQ

(VCC – VEE), or 2

× I

× V

For the AD8016 powered on +12 V and –12 V supplies (

±V

S),

the number is 0.6 W. In a differential driver circuit (Figure 6),

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