ISL1591_14 Datasheet, PDF (15/18 Pages) Intersil Corporation – Fixed Gain, Dual Port, VDSL2 Line Driver

ISL1591
1 PORT OF 2
DRAWN
+
+24V
I = 14mA
50
Vdiff
Vi
POWER
SPLITTER
Rg*
¼
ISL1591
-
0V
C0
0V
C1
Rf*
Rp*
Rp*
Rf*
Rb
RL 82.6 Vo
Rb
-
¼
ISL1591
RL = 100 /(1.1)² = 82.6
Vo/Vdiff = 11.6 V/V (21.3dB)
+
50
0V
*Integrated internally
FIGURE 49. ACTIVE TERMINATION TEST CIRCUIT
Figure 49 shows one of two ports configured in an active
termination circuit used for all characterization tests. This is
showing the device operating in the full power mode, but data
has been shown at the other power settings as well.
The 82.6Ω differential load is intended to emulate a 100Ω line
load reflected through a 1:1.1 turns ratio transformer
(100Ω/(1.1²) = 82.6Ω load). The gain and output impedance for
this circuit can be described by the following equations.
The ideal transfer function is set by the open circuit gain
(R_L = infinite) and an equivalent output impedance Z_O.
V_o
V_i
=
A_oc
R_L
R_L + Z_o
(EQ. 1)
The goal of the positive feedback resistor, R_p, is to provide some
“gain” in the apparent output impedance over just the 2*R_b. It
also will act to increase the A_OC over the simple differential gain
equation if a synthesis factor (SF) is defined, as shown in
Equation 2:
SF =
1
R −R
1− ^{f b}
R
p
(EQ. 2)
We can see this "gain" is achieved by letting R_P be > R_F. The
closer R_p is to R_f - R_b, the more "gain" is achieved but at the risk
of instability. Keeping a synthesis factor of < 4 is desirable. With
SF defined in Equation 2, the exact A_OC and Z_O will be as shown
in Equations 3 and 4:
A_oc
=
SF (1+ 2 ^Rf
R_g
+
R _f − R_{m )}
R_p
(EQ. 3)
Z _o = SF (2R_b )
(EQ. 4)
The internal resistors and external R_b resistors shown in
Figure 49 were configured to achieve the following results.
SF = 3.93
A_OC = 20.9V/V
Z_O = 66Ω
Putting these together into the gain to an 82.6Ω load gives the
following test condition as shown by Equation 5.
V_o
V_i
=
A_oc
R_L
R_L + Z_o
= 20.9 ^82.6Ω
82.6Ω + 66Ω
= 11.6_⎜⎛ ^V
⎝^V
⎟⎞
⎠
(EQ. 5)
The advantage offered by this technique is that for any swing
desired at the load, there is less voltage drop through the
physical output matching resistor than if we simply inserted two
33Ω R_b resistors to achieve the 66Ω output impedance achieved
in this test circuit. Any load current required in R_L will rise to the
output pins through 2*R_b. The voltage rise from the load swing to
the output pin swing is given by Equation 6:
R_L + 2R_b
R_L
(EQ. 6)
This was a factor of 1.36 for the test circuit shown in Figure 49.
Hence a ±10V swing at each output in Figure 49 will produce a
40V_P-P differential swing which will drop to the load divided by
1.36 or a 29.41V_P-P differential swing at the load.
Distortion and MTPR/MBPR
The ISL1591 is intended to provide very low distortion levels
under the demanding conditions required by the discrete
multi-tone (DMT) characteristic of modern DSL modulations. The
standard test for linearity is the Multi-Tone Power Ratio (MTPR)
test where a specified PSD profile is loaded up with discrete
carriers over the specified frequencies in such a way as to
produce the maximum rated line power and Peak to Average
Ratio (PAR) with some tones missing. The measure of linearity is
the delta between the active tones vs a missing tone. To the
extent that the amplifier is slightly non-linear, it will fold a small
amount of power into the missing tones through intermodulation
products for the active tones. Missing band power ratio (MBPR) is
a similar measurement test comparing the added non-linearity in
the missing frequency bands to the nearest tone. Any
non-linearity in the missing band will affect the receive path
performance in a DSL system. Figure 36 shows the circuit
operating at the low power setting used to test 8b VDSL2
frequency plan and power. For this test, the carriers are spaced
at 5kHz.
This -62dBc average MBPR is exceptional for the very low 7mA
total quiescent current used in this configuration. Operating at
reduced power targets on the line will improve MBPR.
When operating in full power mode of 14mA of total quiescent
current, ISL1591 can deliver better than -60dBc average MBPR
for 30a VDSL2 upstream band (US3), as shown in Figure 32.
15
FN7625.1
October 31, 2012