LT6300 Datasheet, PDF (12/16 Pages) Linear Technology – 500mA, 200MHz X DSL LINE DRIVER IN 16-LEAD SSOP PACKAGE

LT6300
APPLICATIO S I FOR ATIO
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_F/R_G at high frequency. The DC
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_F/R_G at
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_OS of the amplifiers, or just
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 (50Ω, 75Ω, 100Ω, 135Ω, etc.).
+
V_i
–
R_F
R_G
C_C
V_O
V_I
=
1
(LOW
FREQUENCIES)
V_O
=
1
+
R_F
R_G
(HIGH
FREQUENCIES)
R_G ≤ R_F/9
1 _{< 5MHz}
2πR_GC_C
6300 F10
Figure 10. Alternate Noninverting Compensation
V_I
R_C
C_C
R_G
C_BIG
+
–
R_F
V_O
V_O
V_I
= 1 AT LOW FREQUENCIES
=
1
+
R_F
R_G
AT
MEDIUM
FREQUENCIES
= 1 + ^RF AT HIGH FREQUENCIES
(R_C || R_G)
6300 F11
Figure 11. Combination Compensation
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_BT can be reduced by a factor
+
V_I
–
R_F
R_G
CABLE OR LINE WITH
CHARACTERISTIC IMPEDANCE R_L
R_BT
V_O
R_L
R_BT = R_L
V_O
V_I
=
1
2
(1
+
R_F/R_G)
6300 F12
Figure 12. Standard Cable/Line Back Termination
R_P2
R_P1
+
V_I
V_P
–
R_F
V_A R_BT V_O
R_L
6300 F13
R_G
FOR
R_BT
=
R_L
n
( )( ) 1 + RF
R_G
R_P1
R_P1 + R_P2
1
=1– _n
R_P2/(R_P2 + R_P1)
V_O
V_I
=
( )^{1 + 1/n}
1 + ^RF
R_G
–
R_P1
R_P2 + R_P1
Figure 13. Back Termination Using Postive Feedback
12