FEATURES
Ideal xDSL Line Drive Amplifier for USB, PCMCIA, or
PCI-Based Customer Premise Equipment (CPE). The
AD8018 provides maximum reach on 5 V supply,
driving 16 dBm of power into a back-terminated,
transformer-coupled 100 ⍀ while maintaining –82 dBc
of out-of-band SFDR.
Rail-to-Rail Output Voltage and High Output Current
Drive
400 mA Output Current into Differential Load of 10 ⍀
@ 8 V p-p
Low Single-Tone Distortion
–86 dBc Worst Harmonic, 6 V p-p into Differential 10 ⍀
@ 100 kHz
Low Noise
4.5 nV/√Hz Voltage Noise Density, 100 kHz
Out-of-Band SFDR = –82 dBc, 144 kHz to 500 kHz,
= 12.5 ⍀, P
R
LOAD
Low-Power Operation
3.3 V to 8 V Power Supply Range
Two Logic Bits for Standby and Shutdown
Low Supply Current of 9 mA/Amplifier (Typ)
Current Feedback Amplifiers
High Speed
The AD8018 is intended for use in single-supply (5 V) xDSL
modems where high-output current and low distortion are
essential to achieve maximum reach. The dual high-speed
amplifiers are capable of driving low distortion signals to within
0.5 V of the power supply rail. Each amplifier can drive 400 mA
of current into 10 Ω (differential) while maintaining –82 dBc
out-of-band SFDR. The AD8018 is available with flexible standby
and shutdown modes. Two digital logic bits (PWDN1 and
PWDN0) may be used to put the AD8018 into one of three
modes: full power, standby (outputs low impedance), and
shutdown (outputs high impedance).
Fabricated with ADI’s high-speed XFCB (eXtra Fast Complementary Bipolar) process, the high bandwidth and fast slew rate
of the AD8018 keep distortion to a minimum, while dissipating a minimum of power. The quiescent current of the AD8018
is a low 9 mA/amplifier. The AD8018 drive capability comes in
compact 8-lead Thermal Coastline SOIC and 14-lead TSSOP
packages. Low-distortion, rail-to-rail output voltage, and highcurrent drive in small packages make the AD8018 ideal for use
in low-cost USB, PCMCIA, and PCI Customer Premise Equipment for ADSL, SDSL, VDSL, and proprietary xDSL systems.
Both models will operate over the temperature range –40°C to
+85°C.
14-Lead TSSOP
NC
2
OUT1
3
–IN1
ⴙIN1
4
5
–V
S
6
PWDN1
7
NC
NC = NO CONNECT
AD8018ARU
81
9
10
11
12
13
14
NC
ⴙV
S
OUT2
–IN2
ⴙIN2
PWDN0
DGND
–60
SFDR – dBc
–70
–80
–90
4186810121416
P
– dBm
LINE
VS = 8V
Figure 1. Out-of-Band SFDR vs. ADSL Upstream Line Power;
= 5 V, N = 4 Turns, 144 kHz to 500 kHz. See Evaluation
V
S
Board Schematics in Figure 11.
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
5V
750⍀
1nF
750⍀
750⍀
10⍀
10⍀
1nF
10k⍀
R1
3.1⍀
P
OUT
16dBm
R2
3.1⍀
TRANSFORMER
10k⍀
1:4
RL = 100⍀
LINEPOWER
13dBm
0.01F
V
IN
0.01F
10k⍀
10k⍀
100⍀
V
REF
0.01F
100⍀
Figure 2. Single-Supply Voltage Differential Drive Circuit
for xDSL Applications
Storage Temperature Range RU, R . . . . . . . –65°C to +125°C
Operating Temperature Range . . . . . . . . . . . –40°C to +85°C
Lead Temperature Range (Soldering 10 sec) . . . . . . . . . 300°C
NOTES
1
Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
AD8018AR–REEL–40°C to +85°C 8-Lead SOICSO-8
AD8018ARU–40°C to +85°C 14-Lead PlasticRU-14
TSSOP
AD8018ARU–REEL –40°C to +85°C 14-Lead PlasticRU-14
TSSOP
AD8018ARU–EVALEvaluation Board RU-14
MAXIMUM POWER DISSIPATION
The maximum power that can be safely dissipated by the AD8018
is limited by the associated rise in junction temperature. The
maximum safe junction temperature for plastic encapsulated
devices is determined by the glass transition temperature of the
plastic, approximately 150°C. Temporarily exceeding this limit
may cause a shift in parametric performance due to a change
in the stresses exerted on the die by the package. Exceeding a
junction temperature of 175°C for an extended period can result
in device failure.
While the AD8018 is internally short circuit protected, this may
not be sufficient to guarantee that the maximum junction temperature (150°C) is not exceeded under all conditions. To ensure
proper operation, it is necessary to observe the maximum power
derating curves.
2.0
1.5
8-LEAD SOIC PACKAGE
1.0
14-LEAD TSSOP PACKAGE
0.5
MAXIMUM POWER DISSIPATION – Watts
0
–40 –30 –20 –100 10 2030 4050 6070 8090
–50
AMBIENT TEMPERATURE – ⴗC
TJ = 150ⴗC
Figure 3. Plot of Maximum Power Dissipation vs.
Temperature
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD8018 features proprietary ESD protection circuitry, permanent damage may occur on
devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.
REV. 0
–3–
Page 4
AD8018
1
FREQUENCY – Hz
10
10
1
100
1000
1001k10k100k1M
0.1
10
100
VS = ⴞ2.5V
R
L
= 100⍀
V
NOISE
ⴙI
NOISE
ⴚI
NOISE
V
NOISE
– nV/ Hz (RTI)
I
NOISE
– pA/ Hz
–Typical Performance Characteristics
ⴙV
750⍀
S
750⍀
AD8018
V
SIGNAL
50⍀
–V
S
TPC 1. Single-Ended Test Circuit
150
100
50
0
–50
OUTPUT VOLTAGE – mV
–100
10F
10F
TANT
0.1F
0.1F
TANT
G = 2
V
= ⴞ2.5V
S
R
= 5⍀
L
R
LOAD
V
OUT
TPC 4. I
3k
2.5k
2k
1.5k
1k
OUTPUT IMPEDANCE – ⍀
500
NOISE
and V
vs. Frequency
NOISE
(0,0)
VS=ⴞ2.5V
(1,0)
(1,1)
–150
50
0
150 200 250 300 350 400 450 500
100
TIME – ns
TPC 2. Small Signal Step Response
TPC 5. Output Impedance vs. Frequency, for Full Power,
0
0.01
0.1110100
FREQUENCY – MHz
1k
Standby, and Shutdown Modes
(+0.1%)
(–0.1%)
3
2
1
0
mV
V
– (VINⴛ2)
–1
–2
–3
102030405060708010090
0
OUT
TIME – ns
TPC 6. 0.1% Settling Time
G = 2
V
= ⴞ2.5
S
V
IN
R
L
= 100⍀
= 2V p-p
3
G = 2
V
= ⴞ2.5V
S
= 5⍀
R
L
2
1
0
–1
OUTPUT VOLTAGE – V
–2
–3
0
100
50
150 200 250 300 350 400 450 500
TIME – ns
TPC 3. Large Signal Step Response
–4–
REV. 0
Page 5
5
FREQUENCY – Hz
10k
OUTPUT VOLTAGE – dBv
–25
1M
5
10M100M1G100k
–22
–19
–16
–13
–10
–7
–4
–1
2
G = 2
V
S
= ⴞ2.5
R
L
= 5⍀
FREQUENCY – Hz
100k
CMRR – dB
–70
1M10M100M
–60
–50
–40
–30
–20
–10
1G
G = 2
V
S
= ⴞ2.5V
R
L
= 100⍀
STANDBY
(1,0) or (0,1)
(1,1)
FULL POWER
2
–1
–4
–7
–10
–13
–16
OUTPUT VOLTAGE – dBv
–19
–22
–25
10k
100k
1M
FREQUENCY – Hz
10M100M
TPC 7. Output Voltage vs. Frequency
G = 2
V
= ⴞ2.5V
S
R
= 100⍀
L
AD8018
1G
TPC 10. Output Voltage vs. Frequency
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
OUTPUT SWING – Volts
1.7
1.6
1.5
1
10
TPC 8. Output Swing vs. R
0
–10
–20
–30
–40
ⴚPSRR
–50
PSRR – dB
–60
–70
–80
–90
100k
TPC 9. PSRR vs. Frequency
REV. 0
ⴙSWING
LOAD RESISTANCE – ⍀
ⴙPSRR
100100010k
1M10M
FREQUENCY – Hz
–SWING
VS = ⴞ2.5V
LOAD
G = 2
V
⌬V
R
= ⴞ2.5V
S
= ⴞ1V
S
= 100⍀
L
100M
12
750⍀ 750⍀
NORMALIZED GAIN – dB
–12
–15
–18
–3
–6
–9
9
6
3
0
100k
STANDBY
V
1M
R
L
OUT
(1,0) or (0,1)
(1,1)
FULL POWER
10M100M
FREQUENCY – Hz
V
IN
50⍀
G = 2
= ⴞ2.5V
V
S
= 100⍀
R
L
TPC 11. Small Signal Frequency Response
TPC 12. CMRR vs. Frequency, Full Power, and Standby
Mode
–5–
1G
Page 6
AD8018
VSIG
50⍀
ⴙV
S
100⍀
1/2
500⍀
500⍀
500⍀
25⍀
ⴙ6V
AD8138
ⴚ6V
500⍀
IN
100⍀
AD8018
750⍀
0.1F
500⍀
750⍀
AD8018
1/2
ⴚV
S
10F0.1F
220F
AD9632
R
L
10F0.1F
7.96k⍀ 402⍀
0.1F
ⴙ6V
0.1F
0.1F
ⴚ6V
7.96k⍀
50⍀
OUT
402⍀
TPC 13. Differential Test Circuit
–60
–70
–80
3RD HARMONIC
V
= 6V p–p
OUT
R
= 10⍀
L
V
= ⴞ 2.5V
S
PWDN 1,0 = 1,1
–60
–70
–80
–90
–100
DIFFERENTIAL DISTORTION – dBc
–110
510
2ND HARMONIC
3RD HARMONIC
LOAD RESISTANCE – ⍀
TPC 16. Differential Distortion vs. R
–60
–70
–80
2ND HARMONIC
VS = ⴞ2.5V
G = 4
f
= 100kHz
O
V
= 6V p–p
OUT
LOAD
VS = ⴞ2.5V
R
= 10⍀
L
G = 4
f
= 100kHz
O
PWDN 1,0 = 1,1
100
–90
–100
DIFFERENTIAL DISTORTION – dBc
–110
0.010.1
FREQUENCY – MHz
2ND HARMONIC
1.0
TPC 14. Differential Distortion vs. Frequency
–50
VS = ⴞ2.5V
R
= 3
⍀
L
G
= 4
–60
f
= 100kHz
DIFFERENTIAL DISTORTION – dBc
–100
–110
–70
–80
–90
O
PWDN 1,0 = 1,1
200
300
PEAK OUTPUT CURRENT – mA
3RD HARMONIC
2ND HARMONIC
400500600700800
TPC 15. Differential Distortion vs. Peak Output Current
–90
3RD HARMONIC
–100
DIFFERENTIAL DISTORTION – dBc
–110
3
45 678
OUTPUT VOLTAGE – Volts
TPC 17. Differential Distortion vs. Peak-to-Peak Output
Voltage
DIFFERENTIAL DISTORTION – dBc
–60
–70
–80
–90
–100
–110
2ND HARMONIC
3
45678
OUTPUT VOLTAGE – Volts
VS = ⴞ2.5V
R
= 10⍀
L
G = 4
f
= 100kHz
O
PWDN 1,0 = 1,0 or 0,1
3RD HARMONIC
TPC 18. Differential Distortion vs. Peak-to-Peak Output
Voltage
–6–
REV. 0
Page 7
AD8018
TRANSFORMER TURNS RATIO
P
LINE
– dBm
3.0
16
6
8
10
14
3.24.04.24.44.63.43.63.8
18
12
VS = 4.75
VS = 8.00
VS = 5.00
VS = 3.33
VS = 4.50
4.8
g
16
15
14
13
– dBm
LINE
P
12
11
10
3.0
VS = 5.25
3.24.04.24.44.64.83.4
3.6
TRANSFORMER TURNS RATIO
VS = 4.75
3.8
VS = 5.00
TPC 19. Line Power vs. Turns Ratio; MTPR = –65 dBc,
f = 43 kHz
–20
–30
–40
–50
MTPR – dBc
–60
–70
–80
34
TRANSFORMER TURNS RATIO – N
VS = 5V
R
LINE
f = 93kHz
P = 13dBm
P = 13.5dBm
P = 14dBm
P = 12.5dBm
P = 12dBm
= 100⍀
5
TPC 20. MTPR vs. Turns Ratio
TPC 22. Line Power vs. Turns Ratio; –75 dBc Out-of-Band
SFDR, f = 361 kHz
10M
1M
100k
10k
1k
100
10
TRANSIMPEDANCE – ⍀
1
0.1
0.01
1k10k100k1M10M100M1G
TRANSIMPEDANCE
FREQUENCY – Hz
PHASE
200
150
100
50
0
–50
–100
–150
–200
TPC 23. Open Loop Transimpedance and Phase
rees
PHASE – De
–30
–40
–50
–60
SFDR – dBc
–70
–80
P = 12dBm
–90
3
TRANSFORMER TURNS RATIO – N
4
TPC 21. Out-of-Band SFDR vs. Turns Ratio for Various
VS = 5V
R
LINE
f = 361kHz
P = 12.5dBm
P = 13dBm
P = 13.5dBm
P = 14dBm
= 100⍀
Line Power
REV. 0
20
18
DECREASING
16
14
12
10
TOTAL SUPPLY CURRENT – mA
8
5
6
0.86
0.880.900.920.940.96 0.981.00 1.02
LOGIC 0 TO 1
POWER-DOWN VOLTAGE – Volts
LOGIC 1 TO 0
INCREASING
TPC 24. Power-Up/-Down Threshold Voltage
–7–
Page 8
AD8018
–10
VIN = 2V p-p
G = 2
–20
V
= ⴞ2.5
S
–30
–40
–50
–60
–70
CROSSTALK – dB
–80
–90
–100
–110
RL = 5
SIDE B DRIVEN
100k
TPC 25. Crosstalk vs. Frequency
THEORY OF OPERATION
The AD8018 is composed of two current feedback amplifiers
capable of delivering 400 mA of output current while swinging
to within 0.5 V of either power supply, and maintaining low
distortion. A differential line driver using the AD8018 can provide
CPE performance on a single 5 V supply. This performance is
enabled by Analog Device’s XFCB process and a novel, twostage current feedback architecture featuring a patent-pending
rail-to-rail output stage.
A simplified schematic is shown in Figure 4. Emitter followers
buffer the positive input, V
, to provide low input current and
P
current noise. The low impedance current feedback summing
junction is at the negative input, V
. The output stage is another
N
high-gain amplifier used as an integrator to provide frequency
compensation. The complementary common-emitter output
provides the extended output swing.
A current feedback amplifier’s dynamic and distortion performance
is relatively insensitive to its closed-loop signal gain, which is
a distinct advantage over a voltage-feedback architecture. Figure
5 shows a simplified model of a current feedback amplifier. The
feedback signal is a current into the inverting node. R
is inversely
IN
proportional to the transconductance of the amplifier’s input stage,
g
. Circuit analysis of the pictured follower with gain yields:
mi
T
ZS
VV G
/
OUTIN
=×
TRGR
ZSFIN
()
()
++×
where:
GRR
=+
1
/
FG
R
/
1
S
CR
+
T
()
TT
Ω
T
=
ZS
()
Rg
=≅
1125
INmi
Recognizing that G ⫻ RIN < RF, and that the –3 dB point is set
when T
= RF, one can see that the amplifier’s bandwidth
Z(S)
depends primarily on the feedback resistor. There is a value of
R
below which the amplifier will be unstable, as an actual ampli-
F
fier will have additional poles that will contribute excess phase
shift. The optimum value for R
depends on the gain and the
F
amount of peaking tolerable in the application.
⍀
RL = 5
⍀
SIDE A DRIVEN
RL = 100
SIDE A DRIVEN
1M10M
FREQUENCY – Hz
RL = 100
⍀
SIDE B DRIVEN
⍀
100M
V
P
V
N
1G
V
O
BIAS
Figure 4. Simplified Schematic
+
V
IN
–
G = 1
V
I
IN
IT = IINCTR
R
F
R
IN
R
G
O
T
+
V
OUT
–
Figure 5. Model of Current Feedback Amplifier
FEEDBACK RESISTOR SELECTION
In current feedback amplifiers, selection of the feedback and gain
resistors will impact on the MTPR performance, bandwidth,
noise, and gain flatness. Care should be exercised in the selection
of these resistors so that the optimum performance is achieved.
Table I shows the recommended resistor values for use in a variety
of gain settings for the test circuit in TPC 1. These values are
intended to be a starting point when designing for any application.
Two digitally programmable logic pins, PWDN1 and PWDN0,
are available on the TSSOP-14 package to select among three
different modes of operation, full power, standby and shutdown.
The DGND pin is the logic ground reference. The logic threshold voltage is established 1 V above DGND. In a typical 5 V
single-supply application, the DGND pin is connected to analog
ground. If PWDN1, PWDN0, and DGND are left unconnected,
the AD8018 will operate at full power.
To operate an amplifier on a single voltage supply, a voltage
midway between the supply and ground must be generated to
properly bias the inputs and the outputs.
A voltage divider can be created with two equal value resistors
(Figure 7). There is a trade-off between the power consumed by
the divider and the voltage drop across these resistors due to the
positive input bias currents. Selecting 2.5 kΩ for R1 and R2 will
create a voltage divider that draws only 1 mA from a 5 V supply.
The voltage generated with this topology can vary due to the
temperature coefficient (TC) of resistance. Resistors that are
closely matched and have a low TC will minimize variations in
the voltage reference due to temperature. One should also be
sure to use a decoupling capacitor (0.1 µF) at the node where
is generated.
V
REF
5V
POWER SUPPLY AND DECOUPLING
The AD8018 can be powered with a good quality (i.e., low-noise)
supply anywhere in the range from 3.3 V to 8 V. However, in
order to optimize the ADSL upstream drive capability to +13 dBm
and maintain the best Spurious Free Dynamic Range (SFDR),
the AD8018 circuit should be supplied with a well regulated 5 V
supply. The 5 V supplied at the USB port may be poorly regulated. Improving the quality of the 5 V supply will optimize the
performance of the AD8018 in a USB-supplied CPE ADSL
modem. This can be accomplished through the use of a step-up
dc-to-dc converter or switching power supply followed by a low
dropout (LDO) regulator such as the ADP3331 (see Figure 6).
Setting R1 to be 953 kΩ and R2 to be 301 kΩ will result in a
of 5 V.
V
OUT
Careful attention must be paid to decoupling the power supply
pins at the output of the dc-to-dc converter, the output of the
LDO regulator and the supply pins of the AD8018. High-quality
capacitors with low equivalent series resistance (ESR) such as
multilayer ceramic capacitors (MLCCs) should be used to minimize supply voltage ripple and power dissipation. A large, usually
tantalum, 10 µF to 47 µF capacitor located in proximity to the
AD8018 is required to provide good decoupling for lower frequency signals. In addition, 0.1 µF MLCC decoupling capacitors
should be located as close to each of the power supply pins as is
physically possible, no more than 1/8 inch away. An additional
large (4.7 µF to 10 µF) tantalum capacitor should be placed on the
board near the supply terminals to supply current for fast, largesignal changes at the AD8018 outputs.
R1
2.5k⍀
2.5k⍀
R2
0.1F
V
REF
Figure 7. Midsupply Reference
DIFFERENTIAL TESTING
The test circuit shown in TPC 13 is used for measuring the differential distortion of the AD8018. A single-ended test signal is
applied to the inverting input of the AD8138 differential driver
with the noninverting input grounded. Applying the differential
output of the AD8138 through 100 Ω resistors serves to isolate
the inputs of the AD8018 differential driver and provide a wellbalanced low-distortion input signal. The differential load (R
)
L
of the AD8018 can be set to the equivalent of the line impedance reflected through a transformer. The AD9632 converts
the differential output voltage back to a single-ended signal.
The differential-to- single-ended converter using the AD9632
has an attenuation of –26 dB and is wired with precision resistors to optimize the balance of differential input signal. The
resulting smaller output signal can be easily measured using a
50 Ω spectrum analyzer.
REV. 0
–9–
Page 10
AD8018
This circuit requires significant power supply bypassing. The
AD8018 operates on a split supply in this circuit. The bypassing
technique shown in TPC 13 utilizes a 220 µF tantalum capacitor
and a 0.1 µF ceramic chip capacitor in parallel, connected from
the positive to negative supply, and a 10 µF tantalum and 0.1 µF
ceramic chip capacitor in parallel, connected from each supply to
ground. The capacitors connected between the power supplies
serve to minimize any voltage ripples that might appear at the
supplies while sourcing or sinking any large differential current.
The large capacitor has a pool of charge instantly available for
the AD8018 to draw from, thus preventing any erroneous distortion results.
POWER DISSIPATION
It is important to consider the total power dissipation of the
AD8018 in order to properly size the heat sink area of an
application. Figure 8 is a simple representation of a differential
driver. With some simplifying assumptions we can estimate the
total power dissipated in this circuit. If the output current is
large compared to the quiescent current, computing the dissipation in the output devices and adding it to the quiescent power
dissipation will give a close approximation of the total power
dissipation in the package. A factor α (~0.6-1) corrects for the
slight error due to the Class A/B operation of the output stage.
It can be estimated by subtracting the quiescent current in the
output stage from the total quiescent current and ratioing that
to the total quiescent current. For the AD8018, α = 0.833.
1
PV rms VV rms
=×++408
TOTOSO
(.–)α
2
R
L
IV P
2
Q SOUT
For the AD8018, operating on a single 5 V supply and delivering a total of 16 dBm (13 dBm to the line and 3 dBm to the
matching network) into 12.5 Ω (100 Ω reflected back through
a 1:4.0 transformer plus back termination), the power is:
= 261 mW + 40 mW
= 301 mW
Using these calculations, and a θ
of 115°C/W for the TSSOP
JA
package and 100°C/W for the SOIC, Tables III and IV show
junction temperature versus power delivered to the line for several supply voltages.
Table III. Junction Temperature vs. Line Power and
Running the AD8018 at voltages near 8 V can produce junction
temperatures that exceed the thermal rating of the TSSOP packages and should be avoided. The shaded areas indicate junction
temperatures greater than 150°C.
LAYOUT CONSIDERATIONS
As is the case with all high-speed applications, careful attention
to printed circuit board layout details will prevent associated
board parasitics from becoming problematic. Proper RF design
technique is mandatory. The PCB should have a ground plane
covering all unused portions of the component side of the board
to provide a low-impedance return path. Removing the ground
plane on all layers from the area near the input and output pins
will reduce stray capacitance, particularly in the area of the
inverting inputs. Signal lines connecting the feedback and gain
resistors should be as short as possible to minimize the inductance
and stray capacitance associated with these traces. Termination
resistors and loads should be located as close as possible to their
respective inputs and outputs. Input and output traces should
be kept as far apart as possible to minimize coupling (crosstalk)
though the board. Adherence to stripline design techniques for
long signal traces (greater than about 1 inch) is recommended.
REV. 0
Page 11
AD8018
Following these generic guidelines will improve the performance
of the AD8018 in all applications.
To optimize the AD8018’s performance as an ADSL differential
line driver, locate the transformer hybrid near the AD8018 drivers
and as close to the RJ11 jack as possible. Maintain differential
circuit symmetry into the differential driver and from the output
of the drivers through the transformer-coupled output of the bridge
circuit as much as possible.
CPE ADSL Application
The low-cost, high-output current dual AD8018 xDSL driver
amplifiers have been specifically designed to drive high fidelity
xDSL signals to within 0.5 V of the power rails, the performance
needed to provide CPE ADSL on a single 5 V supply. The
AD8018 may be used in transformer-coupled bridge hybrid circuits to drive modulated signals including Discrete MultiTone
(DMT) upstream to the central office.
Evaluation Board
The AD8018ARU-EVAL evaluation board circuit in Figure 12
offers the ability to evaluate the AD8018 in a typical xDSL bridge
hybrid circuit.
The receiver circuit on these boards is typically unpopulated.
Requesting samples of the AD8022AR with the AD8018ARUEVAL board will provide the capability to evaluate the
AD8018ARU along with other Analog Devices products in a typical transceiver circuit. The evaluation circuits have been designed
to replicate the CPE side analog transceiver hybrid circuits.
The circuit mentioned above is designed using a one-transformer
transceiver topology including a line receiver, line driver, line
matching network, an RJ11 jack for interfacing to line simulators,
and transformer-coupled inputs for single-ended-to-differential
input conversion.
AC-coupling capacitors of 0.01 µF, C8, and C10, in combina-
tion with 10 kΩ resistors R24 and R25, will form a zero frequency
at 1.6 kHz.
Transformer Selection
Customer premise ADSL requires the transmission of a +13 dBm
(20 mW) DMT signal. The DMT signal can have a crest factor
as high as 5.3, requiring the line driver to provide peak line power
of 27.5 dBm (560 mW). 27.5 dBm peak line power translates
into a 7.5 V peak voltage on the 100 Ω telephone line. Assuming
that the maximum low-distortion output swing available from
the AD8018 line driver on a 5 V supply is 4 V and, taking into
account the power lost due to the termination resistance, a step-up
transformer with turns ratio of 4.0 or greater is needed.
In the simplified differential drive circuit shown in Figure 2, the
AD8018 is coupled to the phone line through a step-up transformer with a 1:4 turns ratio. R1 and R2 are back-termination
or line-matching resistors, each 3.1 Ω (100 Ω/(2 × 4
100 Ω is the approximate phone line impedance. The total differential load for the AD8018, including the termination resistors,
is 12.5 Ω. Even under these conditions the AD8018 provides low
distortion signals to within 0.5 V of the power rails.
2
)), where
Stability Enhancements
The CPE bridge hybrid circuit presents a complex impedance to
the drive amplifiers, particularly when transformer parasitics are
factored in. To ensure stable operation under the full range of
load conditions, a series R-C network (Zoebel Network) should
be connected between each amplifier’s output and ground. The
recommended values are 10 Ω for the resistor and 1 nF for the
capacitor to create a low impedance path to ground at frequencies above 16 MHz (see Figure 2). R33 and R34 are added to
improve common-mode stability.
Receive Channel Considerations
A transformer used at the output of the differential line driver to
step up the differential output voltage to the line has the inverse
effect on signals received from the line. A voltage reduction
or attenuation equal to the inverse of the turns ratio is realized
in the receive channel of a typical bridge hybrid. The turns ratio
of the transformer may also be dictated by the ability of the receive
circuitry to resolve low-level signals in the noisy twisted pair telephone plant. Higher turns ratio transformers effectively reduce the
received signal-to-noise ratio due to the reduction in the received
signal strength.
The AD8022, a dual amplifier with typical RTI voltage noise of
only 2.5 nV/√Hz and a low supply current of 4 mA/amplifier, is
recommended for the receive channel.
DMT Modulation, MultiTone Power Ratio (MTPR), and
Out-of-Band SFDR
ADSL systems rely on 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. A uniquely encoded, Quadrature Amplitude Modulation (QAM)-like signal occurs at the center
frequency of each subband or tone. See Figure 9 for an example
of a DMT waveform in the frequency domain, and Figure 10 for
a time domain waveform. 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 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 process DMT and other heavily
modulated waveforms. A typical ADSL upstream DMT signal
can contain as many as 27 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 second or
third harmonic distortion components. Measurements of MTPR
are typically made on the line side or secondary side of the
transformer.
REV. 0
–11–
Page 12
AD8018
20
0
–20
–40
POWER – dBm
–60
–80
0150
50
FREQUENCY – kHz
100
Figure 9. DMT Waveform in the Frequency Domain
MTPR versus transformer turns ratio is depicted in TPC 21 and
covers a variety of line power ranging from +12 dBm to +14 dBm.
As the turns ratio increases, the driver hybrid can deliver more
undistorted power due to higher output current capability.
Significant degradation of MTPR will occur if the output of the
driver swings to the rails, causing clipping at the DMT voltage
peaks. Driving DMT signals to such extremes not only compromises “in-band” MTPR, but will also produce spurs that exist
outside of the frequency spectrum containing the desired DMT
power. “Out-of-band” spurious free dynamic range (SFDR) can
be defined as the relative difference in amplitude between these
spurs and a tone in one of the upstream bins. Compromising
out-of-band SFDR is equivalent to increasing near end crosstalk (NEXT). Regardless of terminology, maintaining out-of-band
SFDR while reducing NEXT will improve the overall performance
of the modems connected at either end of the twisted pair.
TPC 21 shows how SFDR varies versus transformer turns ratio
for line power ranging from +12 dBm to +14 dBm. As line
power increases, or turns ratio decreases, SFDR degrades. The
power contained in the spurs can be measured relative to the
power contained in a typical upstream carrier and is expressed
in dBc as SFDR, similar to MTPR.
The supply voltage of the driver can also affect SFDR. As the
supply voltage is increased, voltage swing is increased as well,
resulting in the ability to deliver more power to the line without sacrificing performance. This can be seen in TPC 22. Less
undistorted power is available when lower turns ratio transformers are used due to voltage clipping of the signal.
4
3
2
1
VOLTS
0
–1
–2
–3
–0.25
–0.2 –1.5 –1.0
–0.05
TIME – ms
0
0.051.0
1.5
0.2
Figure 10. DMT Signal in the Time Domain
Generating DMT Signals
At this time, DMT-modulated waveforms are not typically
menu-selectable items contained within AWGs. 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 accomplished using a Tektronics AWG 2021 equipped with Option
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. Note that
the DMT waveforms (available on the Analog Devices website,
http://www.analog.com), or similar .WFM files are needed to
produce the digital data required to drive the TxDAC from the
optional TTL Digital Data output of the TEK AWG2021.
TxDAC is a trademark of Analog Devices, Inc.
–12–
REV. 0
Page 13
AD8018
55
P4 1
P4 2
P4 3
56
R17
2.49k⍀
R16
2.49k⍀
R30
0⍀
V
B
A
B
A
V
R31
0⍀
C20
0.1F
TP10
C8
0.1F
R11
CC
50⍀
C10
0.1F
53
P3 3
P3 2
P3 1
54
R24
10k⍀
R29
10k⍀
3
2
JP4
1
C28
DNI
3
2
JP3
1
R15
50⍀
CC
TP11
V
CC
V
CC
2
0.1F
C5
R8
100⍀
V
CC
2
R14
100⍀
TP17
TP18
R2
750⍀
VCC-R
AGND;4
V
AGND;4
VCC-T
U1
AD8018
R18
750⍀
R19
750⍀
U1
AD8018
VCC-T
VCC-R
;8
CC
+V
10k⍀
10k⍀
+V
U2
AD8022
DNI
R6
DNI
R7
DNI
U2
AD8022
DNI
R33
R34
DNI
TP6
R20
DNI
R3
10⍀
1
R1
10⍀
1WATT
2
R4
10⍀
R21
DNI
TP8
R12
DNI
C1
DNI
R9
DNI
V
CC
2
R5
R10
DNI
C16
DNI
C11
DNI
C22
1000pF
C27
1000pF
C12
DNI
C2
DNI
C18
DNI
C3
DNI
TP7
TP9
TP4
TP5
PR1
PR2
TB1 1
CAPPOLY
CAPPOLY
2TB1
C4
DNI
NC = 5,6
4
2
3
1
C7
DNI
TP23 TP24 TP25 TP26
L5
BEAD
C14
10F
T1
C9
DNI
7
CAPPOLY
9
8
10
C26
0.1F
U2 DECOUPLING
CAPPOLY
C6
DNI
TP1
TP2
C15
0.01F
C23
DNI
R32
DNI
1
2
3
4
5
6
TP19
C17
10F
25V
C19
DNI
25V
78
VCC-T
V
CC
-R
DNI: DO NOT INSTALL
R27
DNI
JP1
R13
DNI
Figure 11. EVAL Board Schematic
V
CC
R28
DNI
100⍀
JP2
R26
100⍀
R25
0.1F
C24
0.1F
C25
PDN0
PDN1
DGND
Figure 12. Input Control Circuit
U1
AD8018
NC1 NC2 NC3
REV. 0
–13–
Page 14
AD8018
Figure 13. Assembly—Primary Side
Figure 14. Silk Screen—Primary Side
–14–
REV. 0
Page 15
Figure 15. Layer 1—Primary Side
AD8018
REV. 0
Figure 16. Layer 2—Ground Plane
–15–
Page 16
AD8018
Figure 17. Layer 3—Power Plane
Figure 18. Layer 4—Secondary Side
–16–
REV. 0
Page 17
AD8018
Figure 19. Assembly—Secondary Side
REV. 0
–17–
Page 18
AD8018
EVALUATION BOARD—BILL OF MATERIALS
Qty.DescriptionVendorRef Desc.
21,000 pF 50 V. 1206 ceramic chip capacitorADS # 4-5-20C22, 27
20.01 µF 50 V. 1206 ceramic chip capacitorADS # 4-5-19C15, 23
50.1 µF 50 V. 1206 size ceramic chip capacitorADS # 4-5-18C5, 20, 24 -26
21.0 µF 16 V. 1206 size ceramic chip capacitorNewark # 83F6841C8, 10
4# 26 red (solid) wire jumperADS # 10-14-3C4, 6, 7, 9
310 µF 16 V. ‘C’ size Tantalum chip capacitorADS # 4-7-6C14, 17, 19
1Ferrite bead (with # 22 wire)ADS # 48-1-1L5
110 Ω 5% 3.0 W. metal oxide power resistorD-K # P10W-3BK-NDR1
60 Ω 5% 1/8 W. 1206 size chip resistorADS # 3-18-88C11, 12, R20, 21, 30, 31
210.0 Ω 1% 1/8 W. 1206 size chip resistorADS # 3-18-120R3, 4
249.9 Ω 1% 1/8 W. 1206 size chip resistorADS # 3-14-26R11, 15
5100 Ω 1% 1/8 W. 1206 size chip resistorADS # 3-18-40R 8, 14, 25, 26, 32
22.49 kΩ 1% 1/8 W. 1206 size chip resistorADS # 3-18-71R16, 17
3750 Ω 1% 1/8 W. 1206 size chip resistorADS # 3-18-8R2, 18, 19
210.0 kΩ 0.1% 0805 size chip resistorADS # 3-36-5R33, 34
210.0 kΩ 1% 1/8 W. 1206 size chip resistorADS # 3-18-119R24 and 29
4Test Point (Black) [GND]ADS # 12-18-44TP23–26 (GND.)
2Test Point (Brown)ADS # 12-18-59TP4, 5
3Test Point (Red)ADS # 12-18-43TP17–19
4Test Point (Orange)ADS # 12-18-60TP1, 2, 10, 11
1Test Point (Yellow)ADS # 12-18-32TP3
2Test Point (Blue)ADS # 12-18-62TP6, 8
2Test Point (Green)ADS # 12-18-61TP7, 9
12 × 5-pin strips (1/4 of a 20-pin Samtek ‘SIP’ strip socket)ADS # 11-2-14(T1)
12 Pos. GRAY term. blk. # 25.161.0253 (Newark # 51F4106)ADS# 12-19-10TB1, 2
40.1 inch ctr. shunt Berg # 65474 -001ADS # 11-2-38JP1–4
22 pin gold male header 0.1 inch ctr. Berg # 69157 -102ADS # 11-2-37JP1, 2
4 50 Ω BNC pc mount Telegartner # J01001A1944ADS # 12-6-22S3–6
1AMP# 555154 -1 MOD. JACK (SHIELDED) 6 –6ADS # 12-20-5P1
23-pin gold male header Waldom D-K # WM 2723 -NDADS # 12-3-80JP3, 4
23-pin gold male locking header Waldom # WM 2701 -NDADS # 12-3-79P3, 4
1AD8018ARU ADSL Driver hybridADS # AD8018ARUU1 (D.U.T.)
1AD8018 TSSOP1T Non-Inverting REV. A Evaluation PC boardD C SEval. PC Board
4# 4 –40 × 1/4" panhead ss machine screwADS # 30-1-1
4# 4 –40 × 1/2" threaded alum. standoffsADS # 30-16-2
–18–
REV. 0
Page 19
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
AD8018
0.1574 (4.00)
0.1497 (3.80)
PIN 1
0.0098 (0.25)
0.0040 (0.10)
SEATING
8 Lead SOIC
(SO-8)
0.1968 (5.00)
0.1890 (4.80)
85
1
0.0500 (1.27)
PLANE
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
ALL DIMENSIONS PER JEDEC STANDARDS MS-012 AA
BSC
0.020 (0.51)
0.013 (0.33)
0.2440 (6.20)
0.2284 (5.80)
0.0688 (1.75)
0.0532 (1.35)
0.0098 (0.25)
0.0075 (0.19)
0.0196 (0.50)
0.0099 (0.25)
8ⴗ
0.050 (1.27)
0ⴗ
0.016 (0.40)
ⴛ 45ⴗ
0.177 (4.50)
0.173 (4.40)
0.169 (4.30)
0.059 (1.50)
0.093 (1.00)
0.031 (0.80)
14 Lead TSSOP
(RU-14)
0.201 (5.10)
0.197 (5.00)
0.193 (4.90)
14
PIN 1
0.0256 (0.65)
BSC
0.006 (0.15)
0.002 (0.05)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
0.047 (1.2)
0.0118 (0.30)
0.0075 (0.19)
8
71
MAX
SEATING
PLANE
0.252
(6.40)
BSC
0.008 (0.20)
0.004 (0.09)
C01519–4.5–7/00 (rev. 0)
8ⴗ
0ⴗ
0.030 (0.75)
0.024 (0.60)
0.018 (0.45)
REV. 0
PRINTED IN U.S.A.
–19–
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