OPA2674_17 Datasheet, PDF (17/36 Pages) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

www.ti.com
+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 Electri-
cal and Typical Characteristics is shown in Figure 4. De-
sign considerations for this inverting, bipolar supply config-
uration are covered either in single-supply configuration
(as shown in Figure 3) or in the Inverting Amplifier Opera-
tion 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 trans-
former (here, a step-up transformer with a turns ratio of
1:1.7). The main advantage of this configuration is the re-
duction of even-order harmonic distortion products. Anoth-
OPA2674
SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
er 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 fre-
quency set here at 5kHz. As the upstream signal band-
width starts at 26kHz, this high-pass filter does not gener-
ate 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 differen-
tial amplifier is 8.85. This gain boosts the AFE signal, as-
sumed to be a maximum of 2V_PP, to a maximum of
17.7V_PP.
Refer to the Setting Resistor Values to Optimize Band-
width 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 re-
ceiver. 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)
17