Datasheet

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SBOS270C − AUGUST 2003 − REVISED AUGUST 2008
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17
1/2
OPA2674
+5V
+V
S
V
S
/2
806
Ω
100
Ω
V
O
V
I
57.6
Ω
806
Ω
R
F
453
Ω
R
G
150
Ω
0.1
µ
F
0.1
µ
F
6.8
µ
F
+
0.1
µ
F
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.
1/2
OPA2674
+5V
V
S
/2
806
Ω
V
I
100
Ω
V
O
806
Ω
R
F
453
Ω
R
M
88.7
Ω
6.8
µ
F
+
0.1
µ
F
0.1
µ
F
R
G
113
Ω
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-
er important advantage for ADSL is that each amplifier
needs only half of the total output swing required to drive
the load.
R
G
82.5Ω
2kΩ
2kΩ
1µF
0.1µF
0.1µF
17.4Ω
R
M
100Ω
AFE
2V
PP
Max
Assumed
R
F
324Ω
20Ω
20Ω
R
F
324Ω
1/2
OPA2674
1/2
OPA2674
+12V
1:1.7
17.7V
PP
I
P
= 128mA
I
P
= 128mA
17.4Ω
R
M
+6V
Z
Line
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
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
2n
2
(1)
(2)