CS8420 datasheet(71/80 Pages) CIRRUS

71 / 80 page

CS8420

DS245PP2

ries with each input pin (RXP and RXN) as shown

in Figure 36. However, if a transformer is not used,

high frequency energy could be coupled into the re-

ceiver, causing degradation in analog performance.

Figures 35 and 36 show an optional DC blocking

capacitor (0.1

µF to 0.47 µF) in series with the ca-

ble input. This improves the robustness of the re-

ceiver, preventing the saturation of the transformer,

or any DC current flow, if a DC voltage is present

on the cable.

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-

tion of grounding or capacitively coupling the

shield to the chassis.

In the case of the consumer interface, the standards

call for an unbalanced circuit having a receiver im-

pedance of 75

Ω ±5%. The connector for the con-

sumer interface is an RCA phono socket. The

receiver circuit for the consumer interface is shown

in Figure 37.

The circuit shown in Figure 38 may be used when

external RS422 receivers, optical receivers or other

TTL/CMOS logic outputs drive the CS8420 receiv-

er section.

15.3

Isolating Transformer Requirements

The transformer should be capable of operating

from 1.5 to 14 MHz, which is equivalent to an au-

dio data rate of 25 kHz to 108 kHz after bi-phase

mark encoding. Transformers provide isolation

from ground loops, 60Hz noise, and common mode

noise and interference. One of the important con-

siderations when choosing transformers is mini-

mizing shunt capacitance between primary and

secondary windings. The higher the shunt capaci-

tance, the lower the isolation between primary and

secondary, and the more coupling of high frequen-

cy energy. This energy appears in the form of com-

mon mode noise on the receive side ground and has

the potential to degrade analog performance.

Therefore, for best performance, shielded trans-

formers optimized for minimum shunt capacitance

should be used. See Application Note 134 for a se-

lection of manufacturers and their part numbers.

XLR

Twisted

Pair

110

Ω

110

Ω

CS8420

RXP

RXN

*See Text

Figure 35. Professional Input Circuit

XLR

Twisted

Pair

110

Ω

110

Ω

CS8420

RXP

RXN

0.01 F

*See Text

Figure 36. Transformerless Professional Input Circuit

RCA Phono

RXP

RXN

CS8420

Coax

Ω

0.01 F

Figure 37. Consumer Input Circuit

RXP

RXN

CS8420

0.01 F

TTL/CMOS

Gate

Figure 38. TTL/CMOS Input Circuit

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