UG-317

Evaluation Board User Guide

Rev. 0 | Page 4 of 12

TERMINATION AND PULL-UP/PULL-DOWN

RESISTORS

The evaluation board includes the RT and RT1 footprints for

fitting termination resistors between the A and B receiver inputs

and the Y and Z driver outputs. By default, the board is fitted

with a 120 Ω resistor, RT, between A and B. This resistor should

be removed if the board is connected to a bus that is already

terminated at both ends. For more information about proper

termination, see the AN-960 Application Note, RS-485/RS-422

Circuit Implementation Guide.

Although the ADM2682E/ADM2687E have a built-in receiver

fail-safe for the bus idle condition, there are footprints on the

evaluation board for fitting the R3 and R1 pull-up resistors to

VISO on A and Y, as well as the R4 and R2 pull-down resistors

to GND on B and Z. These resistors can be fitted if the user is

connecting to other parts that require such external biasing resis-

tors on the bus. The exact value required for a 200 mV minimum

differential voltage in the bus idle condition depends on the supply

voltage (for example, 960 Ω for 3.3 V and 1440 Ω for 5 V).

For more information about the bus idle fail-safe, see the AN-960

Application Note, RS-485/RS-422 Circuit Implementation Guide.

DECOUPLING AND RESERVOIR CAPACITORS

The evaluation board uses the following decoupling and

reservoir capacitors:

•

On the logic side of the board, the C1 and C2 capacitors

should be 10 μF and 100 nF ceramic capacitors, respectively,

and the C4 and C6 capacitors should be 10 nF and 100 nF

ceramic capacitors, respectively.

•

On the bus side of the board, the C5 and C7 capacitors

should be 10 nF and 100 nF, respectively, and the C8 and

C9 capacitors should be 100 nF and 10 μF, respectively.

BOARD INTERNAL LAYER THICKNESS

The ADM2682E/ADM2687E evaluation board consists of

six layers with four internal layers. The spacing between the

internal board layers was chosen as specified in Table 2 to

maximize the stitching capacitance on the board.

Table 2. Spacing Between Layers of the Evaluation Board

Layers

Thickness of Space Between Layers (mm)

1 to 2

0.1016

2 to 3

0.2032

3 to 4

0.2032

4 to 5

0.2032

5 to 6

0.1016