OPA2613_14 Datasheet, PDF (18/34 Pages) Texas Instruments – Dual, Wideband, High Output Current, Operational Amplifier with Current Limit

OPA2613
SBOS249H − JUNE 2003 − REVISED AUGUST 2008
SINGLE-SUPPLY ADSL UPSTREAM DRIVER
Figure 6 shows an example of a single-supply ADSL
upstream driver. The dual OPA2613 is configured as a
differential gain stage to provide signal drive to the primary
of the transformer (here, a step-up transformer with a turns
ratio of 1:2). 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.
+12V
0.1µF
20Ω
1/2
OPA2613
R_F
1kΩ
I_P = 150mA
R_M
12.5Ω _1:n
AFE
2V_PP
+6.3V
1kΩ
Max
Assumed 0.1µF ^1kΩ
R_G
308Ω
1µF
R_F
1kΩ
15V_PP
R_M
12.5Ω
Z_LINE
100Ω
20Ω
1/2
OPA2613
I_P = 150mA
Figure 6. Single-Supply ADSL Upstream Driver
The analog front-end (AFE) signal is AC-coupled to the
driver, and the noninverting input of each amplifier is
biased slightly above the mid-supply voltage (+6.3V 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 1.6kHz. As the
upstream signal bandwidth starts at 26kHz, this high-pass
filter does not generate any problems and has the
advantage of filtering out unwanted lower frequencies.
The input signal is amplified with a gain set by the following
equation:
G_D
+
1)²
R_F
R_G
(3)
With R_F = 1kΩ and R_G = 308Ω, the gain for this differential
amplifier is 7.5. This gain boosts the AFE signal, assumed
to be a maximum of 2V_PP, to a maximum of 15V_PP.
The two back-termination resistors (12.5Ω each) added at
each input 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
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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
2n²
(4)
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
V _RMS ²
(1mW) R_L
(5)
With P_L power and V_RMS voltage at the load, and R_L load
impedance, this gives the following:
Ǹ _{VRMS +}
(1mW)
R_L
PL
10 10
(6)
V_P + Crest Factor V_RMS + CF V_{RMS (7)}
with V_P peak voltage at the load and CF Crest Factor.
V_LPP + 2 CF V_RMS
(8)
with V_LPP: peak-to-peak voltage at the load.
Consolidating Equations 4 through 7 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,
V_LPP + 2
CF
Ǹ_(1mW)
R_L
PL
10 10 ₍₉₎
This V_LPP is usually computed for a nominal line
impedance and may be taken as a fixed design target.
The next step for the driver is to compute the individual
amplifier output voltage and currents as a function of V_PP
on the line and transformer turns ratio. As the turns ratio
changes, the minimum allowed supply voltage changes
along with it. The peak current in the amplifier output is
given by:
"I_P
+
1
2
2 V_LPP
n
1
4R_M
(10)
With V_LPP as defined in Equation 8, and R_M as defined in
Equation 4 and shown in Figure 7.
R_M
1:n
V_pp =
2V_Lpp
n
V_Lpp
n
R_M
R_L
V_Lpp
Figure 7. Driver Peak Output Voltage