AD8016
GENERATING DMT
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 cur-
rent-driving capability and experiences voltage clipping while
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.
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 manufactur-
ers. 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. http://www.analog.com/.
Z2
Z' ≡
2
2 × N
(
)
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.
EVALUATION BOARDS
The AD8016ARP-EVAL, AD8016ARB-EVAL, AD8016ARE-EVAL
boards available through Analog Device 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 circuit (see Figures 46a and
46b) including line matching network, an RJ11 jack for interfac-
ing 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 EVAL 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 down-
stream data in the ...128.wfm files.
4
V
= ؎12V
S
PWDN1,0 = (1,1)
3
2
V
= ؎11.4V
S
PWDN1,0 = (1,1)
V
= ؎12V
S
PWDN1,0 = (1,0)
1
0
V
= ؎11.4V
S
PWDN1,0 = (1,0)
–1
–2
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 Tektronics AWG2021
arbitrary 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 2021. 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.
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
DOWNSTREAM TURNS RATIO
Figure 40. Dynamic Headroom vs. XFMR Turns Ratio,
VS = 12 V
Once an optimum turns ratio is determined, the amplifier will
have 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
performance 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
POWER DISSIPATION
In order to properly size the heat sinking area for your applica-
tion, it is important to consider the total power dissipation of
the AD8016. The dc power dissipation for VIN = 0 is IQ (VCC –
VEE), or 2 × IQ × VS.
PWDN1
PWDN0
PD (W)
MTPR
1
1
0
0*
1
0
1
0
1.454
1.262
1.142
0.120
–78 dBc
–75.3 dBc
–57.2 dBc
N/A
For the AD8016 powered on +12 V and –12 V supplies ( VS),
the number is 0.6 W. In a differential driver circuit (Figure 6),
*This mode is quiescent power dissipation.
REV. A
–13–
ADI [ ADI ]
