73M2901CE Data Sheet
DS_2901CE_031
6 Design Considerations
The 73M2901CE single chip modem includes all the basic modem functions. Programmable configuration
options make this device highly adaptable to a wide variety of applications.
Unlike digital logic circuitry, modem designs must contend with precise frequency tolerances and verify
low-level analog signals to ensure acceptable performance. Using good analog circuit design practices
will generally result in a sound design. The crystal oscillator should be held to a 50 ppm tolerance. The
recommendations in this section should be taken into consideration when starting new designs.
6.1 Layout Considerations
Good analog/digital design rules must be used to control system noise in order to obtain high
performance in modem designs. The more digital circuitry present in the application, the more attention
to noise control is needed.
High speed, digital devices should be locally bypassed, and the telephone line interface and the modem
should be located next to each other near where the telephone line connection is accessed. It is
recommended that power supplies and ground traces be routed separately to the analog and digital portions
on the board. Digital signals should not be routed near low-level or high impedance analog traces.
The 73M2901CE should be considered a high performance analog device. A 3.3 µF electrolytic capacitor in
parallel with a 0.1 µF Ceramic capacitor should be placed between each VPD and VND pin and a 10 µF and
0.1 µF between VPA and VNA. A 0.1 µF ceramic capacitor should be placed between VREF and VNA as
well as between VBG and VNA. Use of ground planes and large traces on power is recommended.
6.2 73M2901CE Design Compatibility
The Teridian 73M2901CE is an enhanced version of the Teridian 73M2901CL and has a number of
additional features. These parts are highly compatible with the earlier 73M2901, however, users should
pay special attention when changing an existing 73M2901 design to use the 73M2901CE or 73M2901CL.
From a hardware standpoint, the key differences involve the User I/O pins USR10 and USR11, the
ASRCH pin and the HBDEN pin. An additional user I/O pin, USR20, replaces the ASRCH pin on the
73M2901CE. This pin may remain safely connected to TXD as long as the host software does not
reconfigure USR20 as an output (S104 bit0=0).
The 73M2901CE contains a high efficiency low power hybrid driver. Due to this enhancement, HBDEN is no
longer required. This pin is an internal no-connect and can safely remain connected to its previous VPD or
GND. The functions of USR10 and USR11 are related to Caller ID and Line In Use/Parallel Pickup support.
Software enhancements to the 73M2901CE are typically achieved by the addition of new AT commands.
The device can be considered a superset of the 73M2901CL and 73M2901C. When converting a design to
the 73M2901CE, it is recommended that the user check the commands and register settings for backward
compatibility to the earlier parts (refer to the 73M2901CE AT Command User Guide for complete details).
6.3 Telephone Line Interface
Transmit levels at the line are dependent on the interface used between the pins and the line. The internal
hybrid line drivers eliminate the need for additional active circuitry to drive the line-coupling transformer.
The analog outputs TXAP and TXAN can be connected directly to the transformer (with the required
impedance matching series resistor or network). Depending upon transformer design (specifically dry
transformers), operation may be affected by the limited amount of DC current generated by the analog
outputs (DC offset). For this reason, Teridian recommends using a coupling capacitor with those
transformers to insure maximum performance.
The line interface circuits shown in Section 7 Reference Designs represent the basic components and
values for interfacing the Teridian 73M2901CE analog pins to the telephone line. The values of these
components have been calculated to minimize the transmission and reception path hybrid losses and are
linked by the following equation: R15=0.242 x R13.
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Rev. 3.3
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
