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LT1886 Fiches technique(PDF) 13 Page - Linear Technology

No de pièce LT1886
Description  Dual 700MHz, 200mA Operational Amplifier
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
Logo LINER - Linear Technology

LT1886 Fiches technique(HTML) 13 Page - Linear Technology

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LT1886
APPLICATIO S I FOR ATIO
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
+
8.45
1k
523
1.21k
1.21k
523
1
µF
–Vi
Vi
AV = 10
8.45
1886 F09
1k
1:1
100
RBT
–Va
Va
–VL
VL
RBT
1886 F10
1:n
RL
+
VBIAS
VRX
+
LT1813
LT1813
RF
RF
RG
RD
RG
RD
RL
n2
= REFLECTED IMPEDANCE
RL
2n2
RL
2n2
= ATTENUATION OF Va
+ RBT
RL
2n2
RL
2n2
RG
RD
=
SET
+ RBT
Figure 9. Power Saving ADSL Modem Driver
Figure 10. Receiver Configuration
Table 2. ADSL Upstream Driver Designs
STANDARD
LOW POWER
Line Impedance
100
100
Line Power
13dBm
13dBm
Peak-to-Average Ratio
5.33
5.33
Transformer Turns Ratio
2
1
Reflected Impedance
25
100
Back-Termination Resistors
12.5
8.35
Transformer Insertion Loss
1dB
0.5dB
Average Amplifier Swing
0.79VRMS
0.87VRMS
Average Amplifier Current
31.7mARMS
15mARMS
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.


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