ETC5054 datasheet(13/18 Pages) STMICROELECTRONICS

13 / 18 page

APPLICATION INFORMATION

POWER SUPPLIES

While the pins at the ETC505X family are well

protected against electrical misuse, it is recom-

mended that the standard CMOS practice be fol-

lowed, ensuring that ground is connected to the

device before any-other connections are made. In

applications where the printed circuit board may

be plugged into a ”hot” socket with power and

clocks already present, an extra long ground pin

in the connector should be used.

All ground connections to each device should

meet at a common point as close as possible to

the GNDA pin. This minimizes the interaction of

ground return currents flowing through a common

bus impedance. 0.1

µF supply decoupling capaci-

tors should be connected from this common

ground point to VCC and VBB as close to the de-

vice as possible.

For best performance, the ground point of each

CODEC/FILTER on a card should be connected

to a common card ground in star formation, rather

than via a ground bus. This common ground point

should be decoupled to VCC and VBB with 10

µF

capacitors.

RECEIVE GAIN ADJUSTMENT

For

applications

where

ETC505X family

CODEC/filter receive output must drive a 600

Ω

load, but a peak swing lower then

± 2.5V is re-

quired, the receive gain can be easily adjusted by

inserting a matched T-pad or

π –pad at the out-

put. Table II lists the required resistor values for

600

Ω terminations. As these are generally non-

standard values, the equations can be used to

compute the attenuation of the closest pratical set

of resistors. It may be necessary to use unequal

values for the R1 or R4 arms of the attenuators to

achieve a precise attenuation. Generally it is tol-

erable to allow a small deviation of the input im-

pedance from nominal while still maintaining a

good return loss. For example a 30dB return loss

against 600

Ω is obtained if the output impedance

of the attenuator is in the range 282

Ω to 319Ω

(assuming a perfect transformer).

Figure 4: T-PAD Attenuator

Figure 5:

Π-PAD Attenuator

Table 2 : Attenuator Tables For

Z1 = Z2 = 300

Ω (all values in Ω).

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.7

3.5

5.2

6.9

8.5

10.4

12.1

13.8

15.5

17.3

34.4

51.3

100

115

129

143

156

168

180

190

200

210

218

233

246

26k

13k

8.7k

6.5k

5.2k

4.4k

3.7k

3.3k

2.9k

2.6k

1.3k

850

650

494

402

380

284

244

211

184

161

142

125

110

3.5

6.9

10.4

13.8

17.3

21.3

24.2

27.7

31.1

34.6

107

144

183

224

269

317

370

427

490

550

635

720

816

924

1.17k

1.5k

52k

26k

17.4k

13k

10.5k

8.7k

7.5k

6.5k

5.8k

5.2k

2.6k

1.8k

1.3k

1.1k

900

785

698

630

527

535

500

473

450

430

413

386

366

= Z1







+ 1

− 1







−2 √

Z1 . Z 2







− 1







= 2

√

Z1.Z 2







− 1







Where: N

√

POWERIN

POWEROUT

an d: S =

√Z1

Also : Z =

√

ZSC . ZOC

Where ZSC = impedance with short circuit termi-

nation and ZOC = impedance with open circuit ter-

mination.

√

Z1 .

Z 2







2 − 1







= Z1







− 1

− 2NS + 1







ETC5054 - ETC5057

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