AN1301 APPLICATION NOTE

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of the board. The signal ground plane may be combined with chassis ground or isolated from it. If the ground

planes are combined, an isolation area is not required. When laying out ground planes, special care must be

taken to avoid creating loop antenna effect. Some guidelines are as follows-

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Run all ground plane as solid square or rectangular regions

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Avoid creating loops with ground planes around other planes

3.4 Recommendations

The following recommendations apply to design and layout of the power and ground planes and will prevent the

most common signal and noise issues.

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Divide the Vcc plane into two sections - analog and digital. The break between the planes should run

under the device.

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When dividing the Vcc plane, it is not necessary to add extra layers to the board. Simply crate moats or

cutout regions in existing layers.

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Place a high-frequency bypass cap (0.1uF) near each analog Vcc pin

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Join the digital and analog sections at one or more points by ferric beads. Ensure that the maximum

current rating of the bead is at least 150% of the nominal current that is expected to flow through it.

(250mA per STE100P)

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Place a bulk capacitor (22uF) on each side of each ferrite bead to stop switching noise from travelling

through the ferrite.

For designs with multiple STE100P’s, it is acceptable to supply all from one analog Vcc plane. This plane can

be joined to the digital Vcc plane at multiple points, with a ferrite bead at each one.

4.0 TWISTED PAIR INTERFACE

4.1 Transmit Interface Circuitry

Figure 2 shows a typical transmit interface circuitry. Current is sourced by the AVddt output to the centertap of

the primary side of the winding. Current flows from the centertap to TX+ and TX-. Other components are as fol-

lows:

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R1 and R2 are 49.9 ohm resistors that provide impedance matching to the line, which has a nominal

impedance of 100 ohm.

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C1 shunts any common-mode energy present in the output to ground.

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The magnetics consists of the main winding and a common-mode choke.

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The common-mode choke stops common mode energy from reaching the line. It works together with

capacitor C1 to direct common-mode energy away from the line.