STA120

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APPENDIX A: RS422 RECEIVER INFORMATION

The RS422 receivers on the STA120 is designed to receive both the professional and consumer interfac-

es, and meet all specifications listed in the digital audio standards. Figure 6 illustrates the internal sche-

matic 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.

Figure 6. RS422 Receiver Internal Circuit

Professional Interface

The digital audio specifications for professional use call for a balanced receiver, using XLR connectors,

with 110

Ω ±20% impedance. (The XLR connector on the receiver should have female pins with a male

shell.) Since the receiver has a very high impedance, a 110

Ω resistor should be placed across the receiv-

er terminals to match the line impedance, as shown in figure 7, and, since the part has internal biasing,

no external biasing network is needed. If some isolation is desired without the use of transformers, a

0.01

µF capacitor should be placed on the input of esch pin (RXP and RXN) as shown in Figure 8. How-

ever, if transformers are not used, high frequency energy could be coupled between transmitter and re-

ceiver causing degradation in analog performance.

Although transformers are not required by AES they are strongly recommended. The EBU requires trans-

formers. Figure 7 and 8 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 connected to the digital audio

receiver. The use of this capacitor is an issue of robustness s the digital audio transmission line does not

have a DC voltage component.

Figure 7. Professional Input Circuit

Figure 8. Transformerless Professional Circuit

K-ix

1K

1K

ix

D98AU983

RXP

RXN

110

Ω

RXP

RXN

(*)See Text

110

Ω

TWISTED

PAIR

D98AU984A

XLR

1

110

Ω

RXP

RXN

(*)See Text

110

Ω

TWISTED

PAIR

D98AU985A

XLR

1

0.01

µF

0.01

µF