CS8412 datasheet(35/38 Pages) CIRRUS

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CS8411 CS8412

DS61F1

APPENDIX A: RS422 RECEIVER

INFORMATION

The RS422 receivers on the CS8411 and CS8412

are designed to receive both the professional and

consumer interfaces, and meet all specifications

listed in the digital audio standards. Figure 20 illus-

trates the internal schematic of the receiver portion

of both chips. The receiver has a differential input.

A Schmitt trigger is incorporated to add hysteresis

which prevents noisy signals from corrupting 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 21, and, since the part has inter-

nal biasing, no external biasing network is needed.

If some isolation is desired without the use of trans-

formers, a 0.01 µF capacitor should be placed on

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

Figure 22. However, if transformers are not used,

high frequency energy could be coupled between

transmitter and receiver causing degradation in an-

alog performance.

Although transformers are not required by AES

they are strongly recommended. The EBU requires

transformers. Figures 21 and 22 show an optional

DC blocking capacitor on the transmission line. A

0.1 to 0.47 µF ceramic capacitor may be used to

block any DC voltage that is accidentally connect-

ed to the digital audio receiver. The use of this ca-

pacitor is an issue of robustness as the digital audio

transmission line does not have a DC voltage com-

ponent.

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-

gous 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-

tion of grounding or capacitively coupling to

ground with a "ground-lift" circuit.

8 k

Ω

4 k

Ω

4 k

Ω

8 k

Ω

16 k

Ω

16 k

Ω

RXP

RXN

Figure 20. RS422 Receiver Internal Circuit

XLR

Twisted

Pair

110

Ω

110

Ω

CS8411/12

RXP

RXN

* See Text

Figure 21. Professional Input Circuit

XLR

Twisted

Pair

110

Ω

110

Ω

CS8411/12

RXP

RXN

0.01

µF

0.01

µF

* See Text

Figure 22. Transformerless Professional Circuit

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