AD816

REV. B

–14–

APPLICATIONS

ADSL Transceiver

The AD816 is designed for the primary purpose of providing an

integrated solution for the transmit and receive functions of an

ADSL modem. ADSL or Asymmetrical Digital Subscriber Line

is a means for delivering up to 6 Mbps from a telephone central

office (CO) into a home over the conventional telephone twisted

pair (local loop) and a few hundred kbps simultaneously in the

opposite direction.

The transmit/receive block is commonly referred to as a hybrid,

which is an old telephone term, and the function was originally

performed with passive circuitry in early phone systems. The

hybrid’s function is to deliver maximum transmit power down

the line, while providing the receive circuitry with a maximum

receive signal and a minimized (self) transmit signal. As the line

gets longer, this separation becomes much more difficult, be-

cause the transmit signal must be larger to reach the other end

with acceptable SNR, while the receive signal is more attenu-

ated by the longer line.

The figure of merit for the performance of the hybrid is com-

monly called trans-hybrid loss and is a measure of how much

the transmit signal that appears in the receive circuit has been

attenuated relative to the amplitude of the transmit signal itself.

It is measured in dBs and is a function of frequency.

In addition to the passive circuits that have been used over time,

active circuit techniques can enhance the hybrid’s performance.

Figure 50 shows one of the various hybrid circuits that uses the

AD816 in an ADSL application. The high power op amps serve

as the transmitter, while the low noise amplifiers serve as the

receiver.

The power amplifiers of the AD816 (D1 and D2) are arranged

in a differential configuration that receives its inputs from the

differential outputs of a D/A converter. The outputs differen-

tially drive the transformer primary with a turns ratio of 1:2.

The line on the secondary side of the transformer has an imped-

ance of 120

Ω. Thus one quarter of this resistance (30 Ω) is

required for back termination on the primary side due to the

impedance scaling by the square of the turns ratio. This resis-

tance is divided in half (15

Ω) and put on each side of the drive

buffers for symmetry (R101 and R201).

The receive section (R1 and R2) is configured as a pair of differ-

ence amplifiers that together produce a differential output that

consists of the receive signal in addition to the transmit signal

attenuated by the trans-hybrid loss.

The circuit is highly symmetrical, so a single-ended explanation

can be easily generalized to understand the differential opera-

tion. D1 output terminals (Pin 6 of the AD816) drives the top

of the primary of T1 through R101. A voltage divider is formed

+15V

0.1 F

10 F

8

6

715

4

5

–15V

7

9

715

11

10

D1

AD816

806

V+

V–

R202

196

R203

196

R204

1.18k

L201

12 H

R201

15

R101

15

2

3

1

R106

348

R105

162

R107

1k

R108

2.37k

RCV OUT+

12

4

10

9

6

8

7

5

T1

XFRMR

C601

0.1 F

C602

0.1 F

TELEPHONE

TWISTED PAIR

D2

0.1 F

10 F

R102

196

R103

196

R104

1.18k

C101

8.2 F

L101

12 H

R1

AD816

13

12

14

R206

348

R205

162

R207

1k

R208

2.37k

RCV OUT–

R2

AD816

C201

8.2 F

Figure 50. AD816 as an ADSL Transceiver