Cirrus Logic CS8427 - Manual (Page 51)

CS8427
DS477F5 51
18.3 AES3 Receiver External
Components
The CS8427 AES3 receiver is designed to accept
both the professional and consumer interfaces.
The digital audio specifications for professional
use call for a balanced receiver, using XLR con-
nectors, 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 input impedance, a 110
Ω resistor should be
placed across the receiver terminals to match the
line impedance, as shown in
Figure 23 . Although
transformers are not required by the AES, they are,
however, strongly recommended.
If some isolation is desired without the use of trans-
formers, a 0.01
μF capacitor should be placed in
series with each input pin (RXP and RXN) as
shown in
Figure 24 . However, if a transformer is
not used, high frequency energy could be coupled
into the receiver, causing degradation in analog
performance.
Figure 23 and Figure 24 show an optional DC
blocking capacitor (0.1 μF to 0.47 μF) in series
with the cable input. This improves the robustness
of the receiver, 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 advan-
tageous to have the ground of two boxes held to
the same potential, and the cable shield might be
depended 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 stan-
dards call for an unbalanced circuit having a re-
ceiver impedance of 75
Ω ±5%. The connector for
the consumer interface is an RCA phono socket.
The receiver circuit for the consumer interface is
shown in
Figure 25 .
The circuit shown in Figure 26 may be used when
external RS422 receivers, optical receivers or oth-
er TTL/CMOS logic outputs drive the CS8427 re-
ceiver section.
18.4 Isolating Transformer Requirements
Please refer to the application note AN134: “AES
and SPDIF Recommended Transformers” for re-
sources on transformer selection
1
XLR
Twisted
Pair
110 Ω
110 Ω
CS8427
RXP
RXN
0.01 F μ
0.01 F μ
* See Text
Figure 23. Professional Input Circuit Figure 24. Transformerless Professional Input Circuit
1
XLR
Twisted
Pair
110 Ω
110 Ω
CS8427
RXP
RXN
0.01 F μ
0.01 F μ
* See Text
RCA Phono
RXP
RXN
CS8427
Coax
75 Ω
75 Ω
0.01 F μ
0.01 F μ
Figure 25. Consumer Input Circuit Figure 26. TTL/CMOS Input Circuit
RXP
RXN
CS8427
0.01 F μ
0.01 F μ
TTL/CMOS
Gate