THS6214IPWPR datasheet(25/41 Pages) TI

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DUAL-SUPPLY VDSL DOWNSTREAM

R =

ZLINE

(3)

LINE DRIVER HEADROOM MODEL

P =10

log

(1mW)

RMS

(4)

RMS

(1mW)

(5)

1.4kW

2kW

0.1 F

10W

100W

LINE

AFE

Max

Assumed

2.2kW

20W

2.2kW

1/4

THS6214

1/4

THS6214

+12V

-12V

1:1.1

I =159mA

10W

2.9kW

V =CrestFactor

=CF

RMS

(6)

RMS

LPP

(7)

LPP

(1mW)

(8)

=1+

(2)

THS6214

www.ti.com ....................................................................................................................................................................................................... SBOS431 – MAY 2009

The two back-termination resistors (RS = 10Ω 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 (RM) 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

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 VLPP = 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:

VLPP is usually computed for a nominal line

impedance and may be taken as a fixed design

target.

With RF = 2.2kΩ and RG = 1.4kΩ, the gain for this

The next step in the design is to compute the

differential amplifier is RP = 2.9kΩ. This gain boosts

individual amplifier output voltage and currents as a

the AFE signal, assumed to be a maximum of 2VPP,

function of peak-to-peak voltage on the line and

to a maximum of 3VPP.

transformer turns ratio.

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