Marine VHF Audio and Signalling Processor
CMX885
5
PCB Layout Guidelines and Power Supply Decoupling
Figure 3 Power Supply and De-coupling
Component Values as per Figure 2.
Notes:
It is important to protect the analogue pins from extraneous inband noise and to minimise the impedance
between the device and the supply and bias de-coupling capacitors. The de-coupling capacitors C3, C7,
C18, C19, C21, C22, C24 and C25 should be as close as possible to the device. It is therefore
recommended that the printed circuit board is laid out with separate ground planes for the AVSS and
DVSS supplies in the area of the CMX885, with provision to make links between them, close to the device.
Use of a multi-layer printed circuit board will facilitate the provision of ground planes on separate layers.
VBIAS is used as an internal reference for detecting and generating the various analogue signals. It must
be carefully decoupled, to ensure its integrity, so apart from the decoupling capacitor shown, no other
loads should be connected. If VBIAS needs to be used to set the discriminator mid-point reference, it must
be buffered with a high input impedance buffer.
The single-ended microphone input and audio output must be ac coupled (as shown), so that their return
paths can be connected to AVSS without introducing dc offsets. Further buffering of the audio output is
advised.
The crystal X1 may be replaced with an external clock source.
© 2010 CML Microsystems Plc
12
D/885/3
CMLMICRO [ CML MICROCIRCUITS ]
