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參數(shù)資料
| 型號： | LMH6678 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Low Power 2-Channel Central-Office xDSL Driver |
| 中文描述： | 低功耗雙通道中環(huán)辦事處的xDSL驅動 |
| 文件頁數(shù)： | 14/17頁 |
| 文件大小： | 1792K |
| 代理商： | LMH6678 |
Application Notes
(Continued)
If additional gain is required, the gain can be increased with
positive feedback using the circuit of
Figure 6
. In this case
the voltage gain A
V
will be
A
V
=5.4*(1-K)/(1-5.4*K)
Where
K = (R
1
||10K)/(R
||10K + R
2
) = 10K*R
1
(10K*R
1
+ R
1
R
2
+
10K*R
2
) and R
1
= R
3
, R
2
= R
4
It is suggested to choose R
<
3K so that the 15% tolerance
of the input resistance will not greatly affect the gain. Fur-
thermore, this circuit will have a differential input resistance
of
R
IN_DIFF
= 2*R
1
2 *R
2
/(4.4)
which may be negative in band. Usually no stability problems
are seen if this |R
| is chosen larger than 500
. To minimize
distortion caused by loading on the Codec outputs, |R
| is
usually chosen to be 1k
or more. Additional blocking ca-
pacitors C
and C
must be inserted in series with R
and R
4
to prevent the circuit from latching. C
and C
should be
chosen to be less than 1/5 of C
1
to avoid large signal
oscillation.
ACTIVE TERMINATION CIRCUIT
The LMH6678 can be used to synthesize the output imped-
ance by using positive feedback to increase the output re-
sistance. In ADSL this technique is often used to lower the
total power consumption of the line driver by reducing the
voltage across the series termination resistors. This ap-
proach gives slightly higher power consumption but better
return loss in the downstream band compared to the R-C
termination of Figure 5. The equations that follow and Figure
7 describe how to implement this technique with the
LMH6678.
1.
Pick positive feedback factor (also called the resistance
gain), A
Z
.
2.
Pick desired output resistance, R
OUT
, seen by the line.
3.
Calculate transformer turns ratio based on A
, line driver
voltage swing, and transformer insertion loss (TIL). R
L
is
the line impedance, 100
for ADSL.
N = [(V
LINEPP
/(2 * 11.2)]* [(1 + R
OUT
/( R
L
* A
Z
)] * 10
(TIL/20)
4.
Calculate R
4
from R
OUT
, A
Z
, and N
R
4
= R
OUT
/ (2 * A
Z
* N
2
)
5.
Calculate the resistance looking into the transformer
secondary (chip side).
R
SEC
= R
L
/ N
2
6.
Calculate K
1
.
K
1
= (A
Z
- 1)/(5.4 * A
Z
)
7.
Calculate K
2
.
K
2
= R
SEC
/( R
SEC
+ 2 * R
4
)
8.
Pick a value for R
2
. Typically 3k
is a good value.
9.
Calculate R
EQ
.
R
EQ
= R
2
/(1-5.4* K
2
)
(Note R
EQ
is usually negative.)
10. Calculate R
IN
R
IN
= [(K
1
*R
2
)/(1-K
1
)*10k]/[10k-(K
1
*R
2
)/(1-K
1
)]
11. Calculate the gain without the input voltage divider.
A
V1
= N*5.4*K
2
* [(R
EQ
//10k)/(R
IN
+ R
EQ
//10k)] /(10
TIL/20
)
12. . Calculate A
VTOTAL
the final required gain from input to
the line.
A
VTOTAL
= V
LINERMS
/V
INRMS
13. . Calculate the voltage divider network of R1 and R3
using A
V1
, transformer insertion loss (TIL),
R
1
= R
IN
* [A
V1
/(A
VTOTAL
*10
TIL/20
)]
R
3
= (2 * R
IN
)/[1 - (A
VTOTAL
*10
TIL/20
)/ A
V1
]
The example shown in Figure 7 is designed to the following
parameters:
V
LINERMS
= 3.13V
(19.8 dBm output power)
A
Z
= 4.5
R
OUT
= 65
(13.5dB return loss)
Crest Factor = 5.8
@
nominal 12V supply
Transformer Insertion Loss = 0.4dB
V
INRMS
= 350mV
(AFE output level)
20084046
FIGURE 6. Increasing Gain
20084047
FIGURE 7. Active Termination Application
L
www.national.com
14
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