REV. B

AD8012

–13–

APPLICATIONS

Line Driving for HDSL

High bitrate digital subscriber line (HDSL) is becoming

popular as a means of providing full duplex data communication at

rates up to 1.544 MBPS or 2.048 MBPS over moderate distances

via conventional telephone twisted pair wires. Traditional T1

(E1 in Europe) requires repeaters every 3,000 feet to 6,000 feet

to boost the signal strength and allow transmission over distances

of up to 12,000 feet. In order to achieve repeaterless transmission

over this distance, an HDSL modem requires a transmitted

power level of 13.5 dBm (assuming a line impedance of 135

Ω).

HDSL uses the two binary/one quaternary line code (2B1Q).

A sample 2B1Q waveform is shown in Figure 5. The digital bit

stream is broken up into groups of two bits. Four analog volt-

ages (called quaternary symbols) are used to represent the four

possible combinations of two bits. These symbols are assigned

the arbitrary names +3, +1, –1, and –3. The corresponding

voltage levels are produced by a DAC that is usually part of an

analog front end circuit (AFEC). Before being applied to the

line, the DAC output is low-pass filtered and acquires the sinu-

soidal form shown in Figure 5. Finally, the filtered signal is

applied to the line driver. The line voltages that correspond to

the quaternary symbols +3, +1, –1, and –3 are 2.64 V, 0.88 V,

–0.88 V, and –2.64 V. This gives a peak-to-peak line voltage of

5.28 V.

VOLTAGE

+3

2.64V

+1

0.88V

–1

–0.88V

–3

–2.64V

SYMBOL

NAME

DAC

OUTPUT

FILTERED

OUTPUT

TO LINE

DRIVER

–1

01

+3

10

+1

11

–3

00

–3

00

+1

11

+3

10

–3

00

–1

01

–1

01

+1

11

–1

01

–3

00

Figure 5. Time Domain Representation of an HDSL Signal

Many of the elements of a classic differential line driver are

shown in the HDSL line driver in Figure 6. A 6 V peak-to-peak

differential signal is applied to the input. The differential gain of

tial output signal is 12 V p-p.

As is normal in telephony applications, a transformer galvani-

cally isolates the differential amplifier from the line. In this case,

a 1:1 turns ratio is used. In order to correctly terminate the line,

it is necessary to set the output impedance of the amplifier to be

equal to the impedance of the line being driven (135

Ω in this

case). Because the transformer has a turns ratio of 1:1, the

impedance reflected from the line is equal to the line impedance

of 135

resistors correctly terminate the line.

6V p-p

12V p-p

1:1

+5V

–5V

750

750

1.5k

1/2

AD8012

1/2

AD8012

0.1 F

0.1 F

66.5

66.5

6V p-p

1:1

135

TO

RECEIVER

CIRCUITRY

TO

RECEIVER

CIRCUITRY

GAIN = +2

UP TO

12,000 FEET

+

–

Figure 6. Differential for HDSL Applications

The immediate effect of back-termination is that the signal from

the amplifier is halved before being applied to the line. This

doubles the power the amplifier must deliver. However, the

back-termination resistors also play an important second role.

Full-duplex data transmission systems like HDSL simulta-

neously transmit data in both directions. As a result, the signal

on the line and across the back termination resistors is the

composite of the transmitted and received signal. The termina-

tion resistors are used to tap off this signal and feed it to the

receive circuitry. Because the receive circuitry “knows” what is

being transmitted, the transmitted data can be subtracted from

the digitized composite signal to reveal the received data.

Driving a line with a differential signal offers a number of

advantages compared to a single-ended drive. Because the two

outputs are always 180 degrees out of phase relative to one

another, the differential signal output is double the output

amplitude of either of the op amps. As a result, the differential

amplifier can have a peak-to-peak swing of 16 V (each op amp

can swing to

±4 V), even though the power supply is ±5 V.

In addition, even-order harmonics (second, fourth, sixth, and

so on.) of the two single-ended outputs tend to cancel out one

another, so the total harmonic distortion (quadratic sum of all

harmonics) decreases compared to the single-ended case, even

as the signal amplitude is doubled. This is particularly advan-

tageous in the case of the second harmonic. Because it is very

close to the fundamental, filtering becomes difficult. In this

application, the THD is dominated by the third harmonic,

which is 65 dB below the carrier (i.e., spurious-free dynamic

range = –65 dBc).

Differential line driving also helps to preserve the integrity of the

transmitted signal in the presence of electromagnetic interfer-

ence (EMI). EMI tends to induce itself equally onto both the

positive and negative signal lines. As a result, a receiver with

good common-mode rejection will amplify the original signal

while rejecting induced (common-mode) EMI.