OPA690 datasheet(18/27 Pages) TI

18 / 27 page

OPA2613

SBOS249D − JUNE 2003− REVISED APRIL 2004

www.ti.com

SINGLE-SUPPLY ADSL UPSTREAM DRIVER

Figure 6 shows an example of a single-supply ADSL

upstream driver. The dual OPA2613 is configured as a

differential gain stage to provide signal drive to the primary

of the transformer (here, a step-up transformer with a turns

ratio of 1:2). The main advantage of this configuration is

the cancellation of all even harmonic distortion products.

Another important advantage for ADSL is that each

amplifier needs only to swing half of the total output

required driving the load.

308

Ω

µF

0.1

µF

0.1

µF

12.5

Ω

100

Ω

ZLINE

AFE

Max

Assumed

Ω

1/2

OPA2613

1/2

OPA2613

+12V

1:n

15VPP

P =150mA

IP =150mA

12.5

Ω

+6.3V

Figure 6. Single-Supply ADSL Upstream Driver

The analog front-end (AFE) signal is AC-coupled to the

driver, and the noninverting input of each amplifier is

biased slightly above the mid-supply voltage (+6.3V 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 1.6kHz. As the

upstream signal bandwidth starts at 26kHz, this high-pass

filter does not generate any problems and has the

advantage of filtering out unwanted lower frequencies.

The input signal is amplified with a gain set by the following

equation:

+ 1 )

With RF = 1kΩ and RG = 308Ω, the gain for this differential

amplifier is 7.5. This gain boosts the AFE signal, assumed

to be a maximum of 2VPP, to a maximum of 15VPP.

The two back-termination resistors (12.5

Ω each) added at

each input 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:

LINE

2n2

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 is done using the following

equations:

+ 10

log

RMS

(1mW)

With PL power and VRMS voltage at the load, and RL load

impedance, this gives the following:

RMS +

(1mW)

+ Crest Factor

RMS + CF

RMS

with VP peak voltage at the load and CF Crest Factor.

LPP

+ 2

RMS

with VLPP: peak-to-peak voltage at the load.

Consolidating Equations 4 through 7 allows expressing

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,

LPP

+ 2

(1mW)

This VLPP is usually computed for a nominal line

impedance and may be taken as a fixed design target.

The next step for the driver is to compute the individual

amplifier output voltage and currents as a function of VPP

on the line and transformer turns ratio. As the turns ratio

changes, the minimum allowed supply voltage changes

along with it. The peak current in the amplifier output is

given by:

+ 1

LPP

With VLPP as defined in Equation 8, and RM as defined in

Equation 4 and shown in Figure 7.

Lpp

pp =

1:n

Figure 7. Driver Peak Output Voltage

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

OPA690 Datasheet(PDF) 18 Page - Texas Instruments

OPA690 Datasheet(HTML) 18 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Datasheet Download

Link URL