AD8012

–12–

REV. A

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 dis-

tances 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

transmitted power level of +13.5 dBm (assuming a line imped-

ance of 135

Ω).

HDSL uses the Two Binary/One Quaternary line code (2B1Q).

A sample 2B1Q waveform is shown in Figure 41. The digital bit

stream is broken up into groups of two bits. Four analogue

voltages (called quaternary symbols) are used to represent the

four possible combinations of two bits. These symbols are as-

signed 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 41. 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 41. Time Domain Representation of a HDSL Signal

Many of the elements of a classic differential line driver are

shown in the HDSL line driver in Figure 42. A 6 V peak-to-

peak differential signal is applied to the input. The differential

differential 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 im-

pedance reflected from the line is equal to the line impedance

of 135

Ω (R

Ω

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 42. 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 com-

posite of the transmitted and received signal. The termination

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 ad-

vantages 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 am-

plitude 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 to this, even-order harmonics (2nd, 4th, 6th, etc.) 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 advantageous for the

case of the second harmonic. As it is very close to the funda-

mental, 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 Electro-Magnetic In-

terference (EMI). EMI tends to induce itself equally on to both

the positive and negative signal line. As a result, a receiver with

good common-mode rejection, will amplify the original signal

while rejecting induced (common-mode) EMI.