13
LT1886
APPLICATIO S I FOR ATIO
WUU
U
Table 2 compares the two approaches. It may seem that
the low power design is a clear choice, but there are further
system issues to consider. In addition to driving the line,
the amplifiers provide back-termination for signals that
are received simultaneously from the line. In order to
reject the drive signal, a receiver circuit is used such as
shown in Figure 10. Taking advantage of the differential
nature of the signals, the receiver can subtract out the
drive signal and amplify the received signal. This method
works well for standard back-termination. If the back-
termination resistors are reduced by positive feedback, a
portion of the received signal also appears at the amplifier
outputs. The result is that the received signal is attenuated
–
+
8.45Ω
1k
523Ω
1.21k
1.21k
523Ω
1µF
–V
i
V
i
A
V
= 10
–
+
8.45Ω
1886 F09
1k
1:1
100Ω
R
BT
–V
a
V
a
–V
L
V
L
R
BT
1886 F10
1:n
R
L
+
–
V
BIAS
V
RX
+
–
LT1813
+
–
LT1813
R
F
R
F
R
G
R
D
R
G
R
D
R
L
n
2
= REFLECTED IMPEDANCE
R
L
2n
2
R
L
2n
2
= ATTENUATION OF V
a
+ R
BT
R
L
2n
2
R
L
2n
2
R
G
R
D
=SET
+ R
BT
Figure 9. Power Saving ADSL Modem Driver Figure 10. Receiver Configuration
Table 2. ADSL Upstream Driver Designs
STANDARD LOW POWER
Line Impedance 100Ω 100Ω
Line Power 13dBm 13dBm
Peak-to-Average Ratio 5.33 5.33
Transformer Turns Ratio 2 1
Reflected Impedance 25Ω 100Ω
Back-Termination Resistors 12.5Ω 8.35Ω
Transformer Insertion Loss 1dB 0.5dB
Average Amplifier Swing 0.79V
RMS
0.87V
RMS
Average Amplifier Current 31.7mA
RMS
15mA
RMS
Peak Amplifier Swing 4.21V Peak 4.65V Peak
Peak Amplifier Current 169mA Peak 80mA Peak
Total Average Power Consumption 550mW 350mW
Supply Voltage Single 12V Single 12V
by the same amount as the reduction in the back-termina-
tion resistor. Taking into account the different transformer
turns ratios, the received signal of the low power design
will be one third of the standard design received signal.
The reduced signal has system implications for the sensi-
tivity of the receiver. The power reduction may, or may not,
be an acceptable system tradeoff for a given design.
Demo Board
Demo board DC304 has been created to provide a versatile
platform for a line driver/receiver design. (Figure 11 shows
a complete schematic.) The board is set up for either single
or dual supply designs with Jumpers 1–4. The LT1886 is
set up for differential, noninverting gain of 3. Each amp is
configured as in Figure 5 for maximum flexibility. The
amplifiers drive a 1:2 transformer through back-termina-
tion resistors that can be reduced with optional positive
feedback. The secondary of the transformer can be iso-
lated from the primary with Jumper 5.
A differential receiver is included using the LT1813, a dual
100MHz, 750V/µs operational amplifier. The receiver gain
from the transformer secondary is 2, and the drive signals
are rejected by approximately a factor of 14dB. Other
optional components include filter capacitors and an RC
snubber network at the transformer primary.
