cx28331 Mindspeed Technologies, cx28331 Datasheet - Page 46

• Manufacturer Part Number: cx28331
• Description: Single/dual/triple E3/ds3/sts-1line Interface Unit
• Manufacturer: Mindspeed Technologies

3.0 Applications
3.1 PCB Design Considerations for the CX2833i
3.1.2.3 Termination
Resistors and Capacitors
3.1.3 Impedance Matching
3.1.4 Other Passive Parts
3.1.5 IBIS Models
3-2
3.1.2.2 VGG Decoupling
It is recommended that the VGG pin be decoupled with a 0.1 µf, 0.01 µF and
0.001 µf capacitors. These capacitors should be placed close to the VGG pin.
The termination resistors and capacitors on the receive RLINE pins should be
placed as close the receiver input on the chip as possible. The series resistors for
the transmit TLINE pins should also be placed as close to the transmitter output
pins as possible, but are less of a priority then the RLINE.
It is critical that both the transmit and receive traces around the transformers and
the matching resistors be kept to a minimum length and that the trace impedance
be matched to 75 ohms.
The transmit signals between the device and the transformer should be routed 75
ohm differentially. The transmit signals should be routed single ended between
the transformer and the BNC connector.
The receive signals should be routed differentially between the transformers and
either differentially or single ended from the transformers to the BNC connectors,
depending on the application. If the application requires ground termination it is
recommended that the signals be routed single ended. If the application does not
require ground termination, then the signals can be routed differentially.
To route signals differentially, the signal pair (positive and negative) should be 75
ohm coupled and should be surrounded by solid power/ground planes (buried
strip line) or be coupled to a power/ground plane (microstrip). Buried strip line is
recommended for internal layers while microstrip line is used for signals routed
on surface layers. There should be no discontinuity in the planes during the path
of the signal traces.
Single ended signals should be 75 ohm coupled between power/ground planes for
inner layers or 75 ohm coupled to a power/ground plane on the outer layers.
There should be no discontinuities in the power/ground planes over the trace path.
Impedance discontinuities occur when a signal passes through vias and travels
between layers. It is recommended to minimize the number of vias and layers
that the transmit/receive signals travel through in the design.
Mindspeed recommends the use of 1:1 transformers for coupling the BNC
connectors to the device. The CX28333 uses six Pulse T3001 transformer devices
to handle the 3 Tx and 3 Rx channels.
It is recommended that a 220 µF tantalum capacitor be used where the power
enters the board.
IBIS (Input/Output Buffer Interface Specification) models for the
CX28331/CX28332/CX28333-1x are available from Mindspeed’s web site
(www.mindspeed.com).
Mindspeed Proprietary and Confidential
Mindspeed Technologies
Single/Dual/Triple E3/DS3/STS-1 Line Interface Unit
™
CX28331/CX28332/CX28333 (-1x)
28333-DSH-001-A