Low Power V.32 bis Modem
CMX869
4.
External Components
R1
X1
C1, C3, C8
C2, C4, C7, C9
C5, C6
100nF
47pF (see text)
100kΩ
6.144MHz
10uF
Resistors ±5%, capacitors ±20% unless otherwise stated.
Figure 2a Recommended External Components for a Typical Application
This device is capable of detecting and decoding small amplitude signals. To achieve this DV , AV
DD
DD
and V
should be decoupled and the receive path protected from extraneous in-band signals. It is
BIAS
recommended that the printed circuit board is laid out with both AV
and DV
ground planes in the
SS
SS
CMX869 area, as shown in Figure 2b, with provision to make a link between them close to the CMX869.
To provide a low impedance connection to ground, the decoupling capacitors (C1 – C4, C7, C8) must be
mounted as close to the CMX869 as possible and connected directly to their respective ground plane.
This will be achieved more easily by using surface mounted capacitors.
V
BIAS
is used as an internal reference for detecting and generating the various analogue signals. It must
be carefully decoupled, to ensure its integrity. If V
needs to be used (other than as shown in figures
BIAS
4a or 4b) to set external analogue levels, it must be buffered with a high input impedance buffer.
The values for capacitors C5 and C6 are suggestions for use with many typical crystals. However the
values of these capacitors must be chosen to comply with the crystal manufacturer’s specification, to
ensure that the clock accuracy is within 100ppm. The DV connections to the Xtal oscillator capacitors
SS
C5 and C6 should also be of low impedance and preferably be part of the DV ground plane to ensure
SS
reliable start up of the oscillator.
© 2004 CML Microsystems Plc
6
D/869/4
CMLMICRO [ CML MICROCIRCUITS ]
