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 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).
0.1F
1/2
UP TO
12,000 FEET
AD8012
+
66.5⍀
R
F
750⍀
R
G
1.5k⍀
R
F
750⍀
12V p-p
6V p-p
135⍀
6V p-p
66.5⍀
1:1
1:1
–
0.1F
1/2
AD8012
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.
–5V
GAIN = +2
TO
RECEIVER
CIRCUITRY
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.
SYMBOL
DAC
VOLTAGE
2.64V
NAME
+3
OUTPUT
FILTERED
OUTPUT
TO LINE
DRIVER
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.
+1
0.88V
–1
–3
–0.88V
–2.64V
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).
–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
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
gain of the amplifier (1+2 RF/RG) is set to +2, so the resulting
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 (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 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.
–12–
REV. A
ADI [ ADI ]
