Datasheet
OPA2677
16
SBOS126I
www.ti.com
SINGLE-SUPPLY ADSL UPSTREAM DRIVER
Figure 6 shows an example of a single-supply ADSL up-
stream driver. The dual OPA2677 is configured as a differen-
tial gain stage to provide signal drive to the primary winding
of the transformer (here, a step-up transformer with a turns
ratio of 1:1.7). The main advantage of this configuration is the
cancellation of all even harmonic distortion products. Another
important advantage for ADSL is that each amplifier needs
only to swing half of the total output required driving the load.
OPA2677 HDSL2 UPSTREAM DRIVER
Figure 7 shows an HDSL2 implementation of a single-supply
upstream driver.
The two designs differ by the values of their matching
impedance, the load impedance, and the ratio turns of the
transformers. All these differences are reflected in the higher
peak current and thus, the higher maximum power dissipa-
tion in the output of the driver.
The analog front end (AFE) signal is AC-coupled to the
driver, and the noninverting input of each amplifier is biased
to the mid-supply voltage (+6V in this case). In addition to
providing the proper biasing to the amplifier, this approach
also provides a high-pass filtering with a corner frequency,
set here at 5kHz. As the upstream signal bandwidth starts at
26kHz, this high-pass filter does not generate any problem
and has the advantage of filtering out unwanted lower fre-
quencies.
The input signal is amplified with a gain set by the following
equation:
G
R
R
D
F
G
=+
•
1
2
(1)
With R
F
= 324Ω and R
G
= 82.5Ω, the gain for this differential
amplifier is 8.85. This gain boosts the AFE signal, assumed
to be a maximum of 2V
PP
, to a maximum of 17.3V
PP
.
Refer to the
Setting Resistor Values to Optimize Bandwidth
section for a discussion on which feedback resistor value to
choose.
The two back-termination resistors (17.4Ω each) added at
each terminal of the transformer make the impedance of the
modem match the impedance of the phone line, and also
provide a means of detecting the received signal for the
receiver. The value of these resistors (R
M
) is a function of the
line impedance and the transformer turns ratio (n), given by
the following equation:
R
Z
M
LINE
n
=
2
2
(2)
R
G
82.5Ω
2kΩ
2kΩ
1µF
0.1µF
0.1µF
R
M
17.4Ω
100Ω
Z
LINE
AFE
2V
PP
Max
Assumed
R
F
324Ω
20Ω
20Ω
324Ω
R
F
1/2
OPA2677
1/2
OPA2677
+12V
1:1.7
17.7V
PP
I
P
= 128mA
I
P
= 128mA
R
M
17.4Ω
+6V
82.5Ω
2kΩ
2kΩ
1µF
0.1µF
0.1µF
R
M
11.5Ω
135Ω
Z
LINE
AFE
2V
PP
Max
Assumed
324Ω
20Ω
20Ω
324Ω
1/2
OPA2677
1/2
OPA2677
+12V
1:2.4
17.3V
PP
I
P
= 185mA
I
P
= 185mA
R
M
11.5Ω
+6V
LINE DRIVER HEADROOM MODEL
The first step in a transformer-coupled, twisted-pair driver
design is to compute the peak-to-peak output voltage from
the target specifications. This is done using the following
equations:
P
V
mW R
L
RMS
L
=•
(
)
•
10
1
2
log
(3)
with P
L
power at the load, V
RMS
voltage at the load, and R
L
load impedance; this gives the following:
VmWR
RMS
L
P
L
=
(
)
••110
10
(4)
V Crest Factor V C V
P
RMS
F
RMS
==••
(5)
with V
P
peak voltage at the load and CF Crest Factor.
V
LPP
= 2 • CF • V
RMS
(6)
with V
LPP
: peak-to-peak voltage at the load.
Consolidating Equations 3 through 6 allows expressing the
required peak-to-peak voltage at the load as a function of the
crest factor, the load impedance, and the power at the load.
Thus,
VCFmWR
LPP L
P
L
=• •
(
)
••21 10
10
(7)
This V
LPP
is usually computed for a nominal line impedance
and may be taken as a fixed design target.
The next step in the design is to compute the individual
amplifier output voltage and currents as a function of V
PP
on
FIGURE 6. Single-Supply ADSL Upstream Driver.
FIGURE 7. HDSL2 Upstream Driver.