OPA4684 Datasheet, PDF (16/26 Pages) Burr-Brown (TI) – Quad, Low-Power, Current-Feedback OPERATIONAL AMPLIFIER

While this circuit is bipolar, using ±5V supplies, it can easily
be adapted to single-supply operation. This is typically done
by providing a supply midpoint reference at the noninverting
inputs then adding DC-blocking caps at each input and in
series with the amplifier gain resistor, R_G. This will add two
real zeroes in the response, transforming this circuit into a
bandpass. Figure 10 shows the frequency response for the
filter of Figure 9.
LOW-POWER DSL TRANSCEIVER INTERFACE
With four amplifiers available, the quad OPA4684 can meet
the needs for both differential driver and receiver in a low-
power xDSL line interface design. Figure 11 shows a simpli-
fied design example. Two amplifiers are used as a noninverting
differential driver while the other two implement the driver
echo cancellation and receiver amplifier function. This ex-
ample shows a single +12V design where the drive side is
taking a 2Vp-p maximum input from the transmit filter
and providing a differential gain of 7, giving a maximum
14Vp-p differential output swing. This is coupled through 50Ω
matching resistors and a 1:1 transformer to give a maximum
7Vp-p on a 100Ω line. This 7Vp-p corresponds to a 10dBm
line power with a 3.5 crest factor.
The differential receiver is configured as an inverting sum-
ming stage where the outputs of the driver are cancelled prior
to appearing at the output of the receive amplifiers. This is
done by summing the output voltages for the drive amplifiers
27
24
21
18
15
12
9
6
3
0
1
10
100
Frequency (MHz)
FIGURE 10. Low-Power, Differential 4th-Order, 100MHz Butterworth.
and their attenuated and inverted levels (at the transformer
input) into the inverting inputs of each receiver amplifier. The
resistor values are set in Figure 11 to give perfect drive signal
cancellation if the drive signal is attenuated by 1/2 going from
the drive amplifier outputs to the transformer input. The
signal received through the transformer has a gain of 1
through the receive amplifiers. Higher gain could easily be
provided by scaling the resistors summing into the inverting
inputs of the receiver amplifiers down while keeping the
same ratio between them.
+12V
AFE
Transmit
Filter
2Vp-p
10kΩ
1/4
OPA4684
900Ω
50Ω
1:1
+6V
V_CM
300Ω
14Vp-p
900Ω
50Ω
10kΩ
1/4
OPA4684
1/4
OPA4684
1.6kΩ
V_CM
1.6kΩ
800Ω
800Ω
100Ω 10dBm
Line 3.5 Crest Factor
Receiver
Filter
800Ω
800Ω
1/4
OPA4684
V_CM
FIGURE 11. Low-Power, XDSL Transceiver Design.
16
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OPA4684
SBOS240B