OPA2674 Datasheet, PDF (17/30 Pages) Burr-Brown (TI) – Dual Wideband, High Output Current Operational Amplifier with Current Limit

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+5V
+V_S
0.1µF
806Ω
+
0.1µF 6.8µF
V_I
57.6Ω 806Ω
1/2
V_O 100Ω
OPA2674
V_S/2
R_F
453Ω
R_G
150Ω
0.1µF
Figure 3. AC-Coupled, G = +4, Single-Supply,
Specification and Test Circuit
The last configuration used as the basis of the +5V
Electrical and Typical Characteristics is shown in Figure 4.
Design considerations for this inverting, bipolar supply
configuration are covered either in single-supply
configuration (as shown in Figure 3) or in the Inverting
Amplifier Operation discussion.
+5V
806Ω
+
0.1µF
6.8µF
806Ω
1/2
OPA2674
V_O 100Ω
V_S/2
R_G
0.1µF _113Ω
V_I
R_M
88.7Ω
R_F
453Ω
Figure 4. AC-Coupled, G = −4, Single-Supply,
Specification and Test Circuit
SINGLE-SUPPLY ADSL UPSTREAM DRIVER
Figure 5 shows a single-supply ADSL upstream driver.
The dual OPA2674 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 1:1.7). The main advantage of this configuration is the
OPA2674
SBOS270 − AUGUST 2003
reduction of even-order harmonic distortion products.
Another important advantage for ADSL is that each
amplifier needs only half of the total output swing required
to drive the load.
0.1µF
20Ω
+12V
1/2
OPA2674
R_F
324Ω
I_P = 128mA
17.4Ω
R_{M 1:1.7}
AFE _+6V
2kΩ
2V_PP
Max
Assumed 0.1µF 2kΩ
R_G
82.5Ω
1µF ^RF
324Ω
17.7V_PP
17.4Ω
R_M
Z_Line
100Ω
20Ω
1/2
OPA2674
I_P = 128mA
Figure 5. 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
to the mid-supply voltage (in this case, +6V). Furthermore,
by providing the proper biasing to the amplifier, this
scheme also provides 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 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
(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 17.7V_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 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
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
(2)
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