AD8012
TO
RECEIVER
CIRCUITRY
APPLICATIONS
Line Driving for HDSL
+5V
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).
0.1ꢁF
1/2
UP TO
12,000 FEET
AD8012
+
66.5ꢂ
R
F
750ꢂ
R
G
1.5kꢂ
R
F
750ꢂ
12V p-p
6V p-p
135ꢂ
6V p-p
66.5ꢂ
1:1
1:1
–
0.1ꢁF
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.
1/2
AD8012
–5V
GAIN = +2
TO
RECEIVER
CIRCUITRY
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.
SYMBOL
DAC
VOLTAGE
2.64V
NAME
+3
OUTPUT
FILTERED
OUTPUT
TO LINE
DRIVER
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.
+1
0.88V
–1
–3
–0.88V
–2.64V
–1 +3
01 10
+1
11
–3
00
–3 +1 +3
00 11 10
–3
00
–1
01
–1 +1
01 11
–1
01
–3
00
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).
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
the amplifier (1+2 RF/RG) is set to +2, so the resulting differen-
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 Ω (RREFL = RLINE/Turns Ratio2). As a result, two 66.5 Ω
resistors correctly terminate the line.
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.
REV. B
–13–
ADI [ ADI ]
