Datasheet

DUAL-SUPPLY VDSL DOWNSTREAM
R =
M
Z
LINE
2n
2
(3)
LINE DRIVER HEADROOM MODEL
P =10 log´
L
(1mW) R´
L
V
RMS
2
(4)
V =
RMS
10
P
L
(1mW) R 10´ ´
L
(5)
R
1.4kW
G
2kW
2kW
0.1 Fm
0.1 Fm
R
10W
S
R
100W
L
Z
LINE
AFE
2V
Max
Assumed
PP
R
2.2kW
F
20W
20W
R
2.2kW
F
1/4
THS6214
1/4
THS6214
+12V
-12V
1:1.1
I =159mA
P
I =159mA
P
R
10W
S
R
2.9kW
P
R
2.9kW
P
V =CrestFactor V =CF V
P
RMS RMS
´ ´
(6)
V =2 CF V´ ´
RMSLPP
(7)
V =2 CF
LPP
´ ´
10
P
L
(1mW) R 10´ ´
L
(8)
G =1+
D
2 R´
F
R
G
(2)
THS6214
www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009
The two back-termination resistors (R
S
= 10 Ω each)
added at each terminal of the transformer make the
Figure 82 shows an example of a dual-supply VDSL
impedance of the modem match the impedance of
downstream driver. Both channels of the THS6214
the phone line, and also provide a means of detecting
are configured as a differential gain stage to provide
the received signal for the receiver. The value of
signal drive to the primary winding of the transformer
these resistors (R
M
) is a function of the line
(in Figure 82 , a step-up transformer with a turns ratio
impedance and the transformer turns ratio (n), given
of 1:1.1). The main advantage of this configuration is
by the following equation:
the cancellation of all even harmonic-distortion
products. Another important advantage for VDSL is
that each amplifier must only swing half of the total
output required driving the load.
The first step in a transformer-coupled, twisted-pair
driver design is to compute the peak-to-peak output
voltage from the target specifications. This calculation
is done using the following equations:
with:
• P
L
= power at the load
• V
RMS
= voltage at the load
• R
L
= load impedance
These values produce the following:
Figure 82. Dual-Supply VDSL Downstream Driver
with:
• V
P
= peak voltage at the load
The analog front-end (AFE) signal is ac-coupled to
• CF = Crest Factor
the driver, and the noninverting input of each
amplifier is biased to the mid-supply voltage (ground
in this case). In addition to providing the proper
with V
LPP
= peak-to-peak voltage at the load.
biasing to the amplifier, this approach also provides a
high-pass filtering with a corner frequency, set here at
Consolidating Equation 4 through Equation 7 allows
5kHz. Because the signal bandwidth starts at 26kHz,
us to express the required peak-to-peak voltage at
this high-pass filter does not generate any problem
the load as a function of the crest factor, the load
and has the advantage of filtering out unwanted lower
impedance, and the power at the load. Thus:
frequencies.
The input signal is amplified with a gain set by the
following equation:
V
LPP
is usually computed for a nominal line
impedance and may be taken as a fixed design
target.
With R
F
= 2.2k Ω and R
G
= 1.4k Ω , the gain for this
The next step in the design is to compute the
differential amplifier is R
P
= 2.9k Ω . This gain boosts
individual amplifier output voltage and currents as a
the AFE signal, assumed to be a maximum of 2V
PP
,
function of peak-to-peak voltage on the line and
to a maximum of 3V
PP
.
transformer turns ratio.
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