LT6300 Linear Technology Corporation, LT6300 Datasheet - Page 12

• Manufacturer Part Number: LT6300
• Description: 500ma, 200mhz Xdsl Line Driver In 16-lead Ssop Package
• Manufacturer: Linear Technology Corporation

C
APPLICATIO S I FOR ATIO
LT6300
Another compensation scheme for noninverting circuits is
shown in Figure 10. The circuit is unity gain at low fre-
quency and a gain of 1 + R
output offset is reduced by a factor of ten. The techniques
of Figures 9 and 10 can be combined as shown in Fig-
ure 11. The gain is unity at low frequencies, 1 + R
mid-band and for stability, a gain of 10 or greater at high
frequencies.
In differential driver applications, as shown on the first
page of this data sheet, it is recommended that the gain
setting resistor be comprised of two equal value resistors
connected to a good AC ground at high frequencies. This
ensures that the feedback factor of each amplifier remains
less than 0.1 at any frequency. The midpoint of the
resistors can be directly connected to ground, with the
resulting DC gain to the V
bypassed to ground with a 1000pF or larger capacitor.
Line Driving Back-Termination
The standard method of cable or line back-termination is
shown in Figure 12. The cable/line is terminated in its
characteristic impedance (50 , 75 , 100 , 135 , etc.).
12
V
BIG
C
I
C
R
R
G
C
C
R
C
V
G
i
Figure 10. Alternate Noninverting Compensation
Figure 11. Combination Compensation
+
–
+
–
R
R
F
F
U
U
F
V
/R
V
O
V
OS
V
O
O
I
2 R
R
G
V
= 1 AT LOW FREQUENCIES
= 1 +
= 1 +
V
G
O
I
at high frequency. The DC
of the amplifiers, or just
1
G
= 1 (LOW FREQUENCIES)
= 1 +
C
R
C
R
(R
F
R
/9
G
F
W
< 5MHz
C
R
R
R
AT MEDIUM FREQUENCIES
|| R
G
F
F
(HIGH FREQUENCIES)
G
)
AT HIGH FREQUENCIES
U
6300
F
/R
F10
6300
G
F11
at
A back-termination resistor also equal to the chararacteristic
impedance should be used for maximum pulse fidelity of
outgoing signals, and to terminate the line for incoming
signals in a full-duplex application. There are three main
drawbacks to this approach. First, the power dissipated in
the load and back-termination resistors is equal so half of
the power delivered by the amplifier is wasted in the
termination resistor. Second, the signal is halved so the
gain of the amplifer must be doubled to have the same
overall gain to the load. The increase in gain increases
noise and decreases bandwidth (which can also increase
distortion). Third, the output swing of the amplifier is
doubled which can limit the power it can deliver to the load
for a given power supply voltage.
An alternate method of back-termination is shown in
Figure 13. Positive feedback increases the effective back-
termination resistance so R
Figure 13. Back Termination Using Postive Feedback
Figure 12. Standard Cable/Line Back Termination
R
V
G
V
I
I
R
+
–
P1
R
G
V
R
P
F
+
–
FOR R
V
V
1 +
O
I
R
=
F
R
R
CHARACTERISTIC IMPEDANCE R
BT
G
F
BT
R
R
1 +
1 + 1/n
=
BT
R
P2
R
P1
R
R
n
V
can be reduced by a factor
V
CABLE OR LINE WITH
R
L
R
R
P2
BT
O
I
G
+ R
F
P1
/(R
=
= R
V
P2
1
2
–
P2
A
L
(1 + R
R
+ R
= 1 –
P2
R
R
BT
P1
+ R
P1
F
)
/R
1
n
R
P1
G
L
)
V
R
O
L
6300
6300
F13
L
V
F12
O