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THS6214 Datasheet, PDF (25/41 Pages) Texas Instruments – Dual-Port, Differential, VDSL2 Line Driver Amplifiers
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THS6214
www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009
DUAL-SUPPLY VDSL DOWNSTREAM
Figure 82 shows an example of a dual-supply VDSL
downstream driver. Both channels of the THS6214
are configured as a differential gain stage to provide
signal drive to the primary winding of the transformer
(in Figure 82, a step-up transformer with a turns ratio
of 1:1.1). The main advantage of this configuration is
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.
AFE
2VPP
Max
Assumed
0.1mF
0.1mF
+12V
20W
1/4
THS6214
RF
2.2kW
IP = 159mA
RS
10W 1:1.1
RP
2kW
2.9kW
RG
1.4kW
RP
2.9kW
2kW
RF
RS
2.2kW
10W
ZLINE
RL
100W
20W
1/4
THS6214
IP = 159mA
-12V
Figure 82. Dual-Supply VDSL Downstream Driver
The analog front-end (AFE) signal is ac-coupled to
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
biasing to the amplifier, this approach also provides a
high-pass filtering with a corner frequency, set here at
5kHz. Because the signal bandwidth starts at 26kHz,
this high-pass filter does not generate any problem
and has the advantage of filtering out unwanted lower
frequencies.
The input signal is amplified with a gain set by the
following equation:
GD = 1 +
2 ´ RF
RG
(2)
With RF = 2.2kΩ and RG = 1.4kΩ, the gain for this
differential amplifier is RP = 2.9kΩ. This gain boosts
the AFE signal, assumed to be a maximum of 2VPP,
to a maximum of 3VPP.
The two back-termination resistors (RS = 10Ω each)
added at each terminal of the transformer make the
impedance of the modem match the impedance of
the phone line, and also provide a means of detecting
the received signal for the receiver. The value of
these resistors (RM) is a function of the line
impedance and the transformer turns ratio (n), given
by the following equation:
RM =
ZLINE
2n2
(3)
LINE DRIVER HEADROOM MODEL
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:
PL = 10 ´ log
VRMS2
(1mW) ´ RL
(4)
with:
• PL = power at the load
• VRMS = voltage at the load
• RL = load impedance
These values produce the following:
VRMS =
(1mW) ´ RL ´ 10
PL
10
(5)
VP = CrestFactor ´ VRMS = CF ´ VRMS
(6)
with:
• VP = peak voltage at the load
• CF = Crest Factor
VLPP = 2 ´ CF ´ VRMS
(7)
with VLPP = peak-to-peak voltage at the load.
Consolidating Equation 4 through Equation 7 allows
us to express the required peak-to-peak voltage at
the load as a function of the crest factor, the load
impedance, and the power at the load. Thus:
VLPP = 2 ´ CF ´
(1mW) ´ RL ´ 10
PL
10
(8)
VLPP is usually computed for a nominal line
impedance and may be taken as a fixed design
target.
The next step in the design is to compute the
individual amplifier output voltage and currents as a
function of peak-to-peak voltage on the line and
transformer turns ratio.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS6214
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