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LT1969 Fiches technique(PDF) 15 Page - Linear Technology

No de pièce LT1969
Description  Dual 700MHz, 200mA, Adjustable Current Operational Amplifier
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
Logo LINER - Linear Technology

LT1969 Fiches technique(HTML) 15 Page - Linear Technology

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LT1969
APPLICATIO S I FOR ATIO
and assuming RP >> RL, we require
Va = Vo (1 – 1/n)
solving
RF/RP = 1 – 1/n
So to reduce the back-termination by a factor of 3 choose
RF/RP = 2/3. Note that the overall gain is increased to:
Vo/Vi = (1 + RF/RG + RF/RP)/[2(1 – RF/RP)]
ADSL Driver Requirements
The LT1969 is an ideal choice for ADSL upstream (CPE)
modems. The key advantages are:
±200mA output drive
with only 1.7V worst-case total supply voltage headroom,
high bandwidth, which helps achieve low distortion, low
quiescent supply current of 7mA per amplifier and a
space-saving, thermally enhanced MS10 package.
An ADSL remote terminal driver must deliver an average
power of 13dBm (20mW) into a 100
Ω line. This corre-
sponds to 1.41VRMS into the line. The DMT-ADSL peak-to-
average ratio of 5.33 implies voltage peaks of 7.53V into
the line. Using a differential drive configuration and trans-
former coupling with standard back-termination, a trans-
former ratio of 1:2 is well suited. This is shown on the front
page of this data sheet along with the distortion perfor-
mance vs line voltage at 200kHz, which is beyond ADSL
requirements. Note that the distortion is better than
–73dBc for all swings up to 16VP-P into the line. The gain
of this circuit from the differential inputs to the line voltage
is 10. Lower gains are easy to implement using the
compensation techniques of Figure 5. Table 2 shows the
drive requirements for this standard circuit.
The above design is an excellent choice for desktop
applications and draws typically 550mW of power. For
portable applications, power savings can be achieved by
reducing the back-termination resistor using positive feed-
back as shown in Figure 9. The overall gain of this circuit
Figure 9. Power Saving ADSL Modem Driver
+
8.45
1k
523
1.21k
1.21k
523
1
µF
–Vi
Vi
AV = 10
8.45
1969 F09
1k
1:1
100
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


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