CS8401 datasheet(28/34 Pages) ETC

28 / 34 page

Appendix A: RS422 Driver Information

The RS422 drivers on the CS8401A and CS8402A

are designed to drive both the professional and con-

sumer interfaces. The AES/EBU specification for

professional/broadcast use calls for a 110

Ω source

impedance and a balanced drive capability. Since the

transmitter impedance is very low, a 110

Ω resistor

should be placed in series with one of the transmit

pins. (A 110

Ω resistor in parallel with the trans-

former would, with the receiver impedance of 110

Ω,

provide a 55

Ω load to the part which is too low.)

The specifications call for a balanced output drive of

2-7 volts peak-to-peak into a 110

Ω load with no ca-

ble attached. Using the circuit in Figure A1, the

output of the transformer is short-circuit protected,

has the proper source impedance, and provides a

5 volt peak-to-peak signal into a 110

Ω load. Lastly,

the two output pins should be attached to an XLR

connector with male pins and a female shell, and

with pin 1 of the connector grounded.

In the case of consumer use, the specifications

call for an unbalanced drive circuit with an out-

put impedance of 75

Ω and a output drive level

of 0.5 volts peak-to-peak

±20% when measured

across a 75

Ω load using no cable. The circuit

shown in Figure A2 only uses the TXP pin and

provides the proper output impedance and drive

level using standard 1% resistors. The connector

for consumer would be an RCA phono socket.

This circuit is also short circuit protected.

The TXP pin may be used to drive TTL or

CMOS gates as shown in Figure A3. This circuit

may be used for optical connectors for digital

audio since they are usually TTL compatible.

This circuit is also useful when driving multiple

digital audio outputs since RS422 line drivers

have TTL interfaces.

The transformer should be capable of operating

from 1.5 to 7 MHz, which is the audio data rate

of 25 kHz to 55 kHz after biphase-mark encod-

ing. Transformers provide isolation from ground

loops, 60 Hz noise, and common mode noise

and interference. One of the important considera-

tions when choosing transformers is minimizing

shunt capacitance between primary and secon-

dary windings. The higher the shunt capacitance,

the lower the isolation between primary and sec-

ondary, and the more coupling of high frequency

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-

f or m er s o p timized f or min imum shunt

capacitance should be used. The following are a

few typical transformers:

Pulse Engineering

Telecom Products Group

7250 Convoy Ct.

San Diego, CA 92111

CS8401/2A

TXN

TXP

110

XLR

CS8401/2A

TXN

TXP

374

90.9

RCA

Phono

Figure A1. Professional Output Circuit

Figure A2. Consumer Output Circuit

CS8401/2A

TXN

TXP

TTL or

CMOS Gate

Figure A3. TTL/CMOS Output Circuit

CS8401A CS8402A

DS60F1

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