ISL1532IRZ Datasheet, PDF (16/19 Pages) Intersil Corporation – Dual Channel Differential DSL Line Driver

ISL1532
TABLE 2. OPTIMUM DRIVER FEEDBACK RESISTOR FOR
VARIOUS GAINS AND SUPPLY VOLTAGES
DRIVER VOLTAGE GAIN
SUPPLY VOLTAGE
5
10
±12V
4k
3k
The ISL1532 features improved frequency compensation for
all power modes and applications, allowing stable operation
at very low power levels and eliminating any need for
external "snubber" circuits. Differential circuits, such as
ADSL2+ line driver applications, can be especially prone to
common-mode oscillation. The ISL1532 is specifically
compensated to eliminate this type of instability and allow for
reliable operation even at very low power levels.
Cable Termination Techniques
The traditional circuit for a line driver with passive
termination is shown in Figure 49. R_BM is the backmatch
resistance added for proper termination at the source. This
backmatch resistance is typically equal to the value of the
cable line characteristic impedance and the load impedance.
The output impedance of the amplifier is negligible in
comparison with the value of the backmatch resistor it
appears in series with. The gain equation reflects the output
voltage across the load resistance with respect to the input
voltage.
R_G
R_F
-
V_IN +
R_BM
Z_L
V_O
R_L
-^V-----^O---
V_IN
=
⎛
⎜1
⎝
+
R^-R----_G^-F--_{⎠⎟⎞}
×
⎛
⎜
⎝
-_R----_B----_M-^R-----^L_+-----R----_L--_{⎠⎟⎞}
FIGURE 49. TRADITIONAL CABLE TERMINATION TECHNIQUE
While functional, this passive termination circuit has some
disadvantages. The output impedance of the driver, while
small, can be a noticeable quantity. The backmatch resistor
is necessary to properly terminate the source end of a
transmission line such as a twisted pair, but now the voltage
delivered to the load is split between that backmatch
resistance and the load resistance. Since there is a required
voltage level at the load, the driver must now produce twice
the voltage swing. The voltage swing and power dissipation
increases. The power burned in the backmatch resistor is
lost as heat which causes the total power dissipation to
doubled. There also is quiescent power used in the op amp.
An alternative technique of cable termination using positive
feedback is shown in Figure 50. With negative feedback
already in place to set the gain, positive feedback can be
used to adjust the output impedance. Lowering the
backmatch resistor without compromising the total source
termination impedance relaxes the output and supply
voltage requirement for the amplifier and reduces the overall
power dissipation. R_P1 and R_P2 are the only additions to the
passive circuit and provide the positive feedback for the
amplifier. This feedback synthesizes larger output
impedance for the amplifier, allowing a reduction of the
backmatch resistance. For convenience, a factor K is being
introduced. It is the ratio between the backmatch resistance
and the physical backmatch resistor.
K
=
----^R-----^L----
RBM
(EQ. 8)
R_G
R_F
R_P1
V_IN
-
+
R_P2
R_BM
Z_L
V_O
R_L
FIGURE 50. ACTIVE TERMINATION TECHNIQUE
The stability of the amplifier and the physical backmatch
resistance tolerance typically limit K to around 4 or 5. The
output impedance of the amplifier is increased by the
positive feedback, allowing the backmatch resistance to
decrease-keeping the total source impedance constant.
R_G
R_F
-
R_BM
+
V_X
R_P1
R_P2
I_X
FIGURE 51. MEASURING OUTPUT IMPEDANCE
Figure 51 shows a standard method for measuring the
output impedance of any circuit. Ohm's law applies, so a
measured voltage (V_X) applied to a node divided by the test
current gives the impedance seen at that node. Ideal op amp
simplifications (input terminals are at the same voltage and
there is no current flowing into the inputs) R_P2 is assumed
much larger the R_BM so the current through the positive
feedback loop can be neglected. The voltage at the input
terminals is given by a resistive divider of the output voltage
on either side of the backmatch resistor. These feedback
resistors alter the output resistance for the op amp, allowing
reduction in the backmatch resistance. The derivations are
as follows:
16
FN6173.3
December 4, 2006