OPA2677IDDA Datasheet, PDF (16/38 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier

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.
0.1µF
+12V
20Ω
1/2
OPA2677
R_F
324Ω
I_P = 128mA
R_M
17.4Ω _1:1.7
AFE
2V_PP
Max
Assumed
+6V
0.1µF
2kΩ
R_G
82.5Ω
2kΩ
1µF
R_F
17.7V_PP
R_M
17.4Ω
324Ω
20Ω
1/2
OPA2677
I_P = 128mA
Z_LINE
100Ω
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.
20Ω
+12V
1/2
OPA2677
0.1µF
324Ω
I_P = 185mA
R_M
11.5Ω _1:2.4
AFE _+6V
2kΩ
2V_PP
Max
Assumed 0.1µF 2kΩ
82.5Ω
1µF
324Ω
17.3V_PP
R_M
11.5Ω
Z_LINE
135Ω
20Ω
1/2
OPA2677
I_P = 185mA
FIGURE 6. Single-Supply ADSL Upstream Driver.
FIGURE 7. HDSL2 Upstream 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_D
= 1+
2 • R_F
R_G
(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_M
=
Z_LINE
2n ²
(2)
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_L
= 10
• log
VRMS ²
(1mW) • R_L
(3)
with P_L power at the load, V_RMS voltage at the load, and R_L
load impedance; this gives the following:
PL
V_RMS = (1mW) • R_L • 10 ¹⁰
(4)
V_P = Crest Factor • V_RMS = C_F • V_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.
PL
Thus,
V_LPP = 2 • CF • (1mW) • R_L • 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
16
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