CS8413 datasheet(34/38 Pages) ETC

34 / 38 page

CS8413 CS8414

DS240F1

APPENDIX A: RS422 RECEIVER

INFORMATION

The RS422 receivers on the CS8413 and CS8414

are designed to receive both the professional and

consumer interfaces, and meet all specifications

listed in the digital audio standards. Figure A20 il-

lustrates the internal schematic of the receiver por-

tion of both chips. The receiver has a differential

input. A Schmitt trigger is incorporated to add hys-

teresis which prevents noisy signals from corrupt-

ing the phase detector.

Professional Interface

The digital audio specifications for professional

use call for a balanced receiver, using XLR connec-

tors, with 110

Ω ±20% impedance. (The XLR con-

nector on the receiver should have female pins with

a male shell.) Since the receiver has a very high im-

pedance, a 110

Ω resistor should be placed across

the receiver terminals to match the line impedance,

as shown in Figure A21, and, since the part has in-

ternal biasing, no external biasing network is need-

ed. If some isolation is desired without the use of

transformers, a 0.01µF capacitor should be placed

on the input of each pin (RXP and RXN) as shown

in Figure A22. However, if transformers are not

used, high frequency energy could be coupled be-

tween transmitter and receiver causing degradation

in analog performance.

Although transformers are not required by AES

they are strongly recommended. The EBU requires

transformers. Figures A21 and A22 show an op-

tional DC blocking capacitor on the transmission

line. A 0.1 to 0.47µF ceramic capacitor may be

used to block any DC voltage that is accidentally

connected to the digital audio receiver. The use of

this capacitor is an issue of robustness as the digital

audio transmission line does not have a DC voltage

component.

Grounding the shield of the cable is a tricky issue.

In the configuration of systems, it is important to

avoid ground loops and DC current flowing down

the shield of the cable that could result when boxes

with different ground potentials are connected.

Generally, it is good practice to ground the shield

to the chassis of the transmitting unit, and connect

the shield through a capacitor to chassis ground at

the receiver. However, in some cases it is advanta-

geous to have the ground of two boxes held to the

same potential, and the cable shield might be de-

pended upon to make that electrical connection.

Generally, it may be a good idea to provide the op-

Ω

16k

Ω

16k

Ω

RXP

RXN

Figure 20. RS422 Receiver Internal Circuit

XLR

110

Ω

Twisted

Pair

* See Text

110

Ω

RXP

RXN

CS8413/14

Figure 21. Professional Input Circuit

XLR

110

Ω

Twisted

Pair

* See Text

110

Ω

RXP

RXN

CS8413/14

0.01 µF

Figure 22. Transformerless Professional Circuit

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