參數(shù)資料
型號(hào)： LTI969CMS
英文描述： Voltage-Feedback Operational Amplifier
中文描述： 電壓反饋運(yùn)算放大器
文件頁(yè)數(shù)： 15/20頁(yè)
文件大?。?/td> 285K
代理商： LTI969CMS
15
LT1969
APPLICATIOU
and assuming R
P
>> R
L
, we require
V
a
= V
o
(1 – 1/n)
solving
R
F
/R
P
= 1 – 1/n
So to reduce the back-termination by a factor of 3 choose
R
F
/R
P
= 2/3. Note that the overall gain is increased to:
V
o
/V
i
= (1 + R
F
/R
G
+ R
F
/R
P
)/[2(1 – R
F
/R
P
)]
W
U
U
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
line. This corre-
sponds to 1.41V
RMS
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 16V
P-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
μ
F
–V
i
V
i
A
V
= 10
–
+
8.45
1969 F09
1k
1:1
100
Table 2. ADSL Upstream Driver Designs
STANDARD
100
13dBm
5.33
2
25
12.5
1dB
0.79V
RMS
31.7mA
RMS
4.21V Peak
169mA Peak
550mW
Single 12V
LOW POWER
100
13dBm
5.33
1
100
8.35
0.5dB
0.87V
RMS
15mA
RMS
4.65V Peak
80mA Peak
350mW
Single 12V
Line Impedance
Line Power
Peak-to-Average Ratio
Transformer Turns Ratio
Reflected Impedance
Back-Termination Resistors
Transformer Insertion Loss
Average Amplifier Swing
Average Amplifier Current
Peak Amplifier Swing
Peak Amplifier Current
Total Average Power Consumption
Supply Voltage
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