AD8017

–12–

REV. C

APPLICATIONS

Output Power Characteristics as Applied to ADSL Signals

The AD8017 was designed to provide both relatively high cur-

rent and voltage output capability. TPCs 12 and 15 quantify the

ac load current versus distortion of the device at loads of 100

Ω

and 25

Ω at 1 MHz. Using approximately –50 dBc as the worst

case distortion limit, the AD8017 exhibits acceptable linearity

to within approximately 1.4 V of either supply rail (12 V or

± 6 V)

while simultaneously providing 200 mA of load current.

These levels are achieved at only 7 mA of quiescent current for

each amplifier.

ADSL applications require signal line powers of 13 dBm that

can randomly peak to an instantaneous power (or V

× I product)

of 28.5 dBm. This equates to peak-to-rms voltage ratio of 5.3-

to-1. Using a 1:2 transformer in the ADSL circuit illustrated

below and 100

Ω as the line resistance, a peak voltage of 4.2 V

at a peak current of 168 mA will be required from the line driver

output (see Table I). See detailed application below. A higher

turns ratio transformer can be used to reduce the primary out-

put voltage swing of the amplifier (for devices that do not have

the voltage swing, but do have the current drive capability).

However, this requires more than an equivalent increase in

current due to the added I

× R losses from the transformer for

the same receiver power. Generally this will result in added

distortion. Table I below shows the ADSL ac current and volt-

ages required for both a 1:1 and 1:2 transformer turns ratio.

1k

1k

0.1 F

0.1 F

8

2

3

1

7

5

6

4.7V

4.7V

1:2

12V

1k

0.1 F

169

169

4

1k

1 F

1 F

0.1 F

10 F

12.5

12.5

100

AD8017

50

EFFECTIVE

LOAD

Figure 7. Single 12 V Supply ADSL Remote Terminal

Transmitter

Table I. DSL Drive Amplifier Requirements for Various Combinations of Line Power, Line Impedance, and Turn Ratios

Line

Insertion

Line

Turns

Crest

Reflected

Per Amp

Peak Per Amplifier Peak Current

Power

Loss

Load

Ratio

Factor

Impedance

R1 = R2

Voltage

Voltage Output

Output

13 dBm

1 dB

100

Ω

1:1

5.3

100

Ω

50

Ω

1.585 V rms

8.4 V peak

84 mA

13 dBm

1 dB

100

Ω

1:2

5.3

25

Ω

12.5

Ω

0.792 V rms

4.2 V peak

168 mA

Single 12 V Supply ADSL Remote Terminal (RT) Transmitter

For consumer use, it is desirable to create an ADSL modem

that can be a plug-in accessory for a PC. In such an application,

the circuit should dissipate a minimum of power, yet still meet

the ADSL specification.

The circuit in Figure 7 shows a single 12 V supply circuit that

uses the AD8017 as a remote terminal transmitter. This supply

voltage is readily available on the PCI connector of PCs. The

circuit configures each half of the AD8017 as an inverter with a

gain of about six. Both of the amplifier circuits are ac coupled at

both the inputs and the outputs. This makes the dc levels of the

circuit independent of the other dc levels of the signal chain.

The inputs will generally be driven by the output of an active

filter, which has a low output impedance. Thus there will be a

minimum of loading of the source caused by the 169

Ω input

impedance in the pass band. The output will require a 1:2 step-

up transformer to drive a 100

Ω line. The reflected impedance

back to the primary will be 25

Ω. With 25 Ω of series termina-

tion added (12.5

Ω in each output), the effective load that the

differential amplifier outputs will drive is 50

Ω.

The input and output ac coupling provides two high pass cir-

cuits. The inputs are formed by the 0.1

µF capacitor and the

169

Ω resistor, which provides a break frequency of about

9.4 kHz. The two 1

µF capacitors in the output along with the

50

Ω effective load provides a 6.4 kHz break frequency in the

output side. Both of these circuits want to reject the Plain Old

Telephone System (POTS) band (dc to 4 kHz) while passing

the ADSL upstream band, which starts at about 20 kHz.

The positive inputs must be biased at midsupply, which is nomi-

nally 6 V. This will maintain the maximum dynamic range of

the output in each direction, regardless of the tolerance of the

supply. The inverting configuration was chosen as this requires

a steady dc current from this supply, as opposed to the signal-

dependent current that would be required in a noninverting

configuration. Several options were studied for creating this supply.

A voltage regulator could be used, but there are several disad-

vantages. The first is that this will not track the middle of the

supplies as it will always have an output that is a fixed voltage

from ground. This also requires an additional active component

that will impact the cost of the total solution.

A two-resistor divider could also be used. There is a tradeoff

required here in the selection of the value of the resistors. As the

resistors become smaller, the amount of power that they will

dissipate will increase. For two 1 k

Ω resistors, the power dissi-

pation in this circuit would be 72 mW. Thus, in order to keep

this power to a minimum, it is desirable to make the resistors as

large as possible.