Preliminary Data Sheet

AD8196

PrA | Page 21 of 24

include external termination resistors for the TMDS differential

pairs on the PCB.

The output termination resistors of the AD8196 back-terminate

the output TMDS transmission lines. These back-terminations

act to absorb reflections from impedance discontinuities on the

output traces, improving the signal integrity of the output traces

and adding flexibility to how the output traces can be routed.

For example, interlayer vias can be used to route the AD8196

TMDS outputs on multiple layers of the PCB without severely

degrading the quality of the output signal.

Auxiliary Control Signals

There are four single-ended control signals associated with each

source or sink in an HDMI/DVI application. These are hot plug

detect (HPD), consumer electronics control (CEC), and two

display data channel (DDC) lines. The two signals on the DDC

bus are SDA and SCL (serial data and serial clock, respectively).

These four signals can be switched through the auxiliary bus of

the AD8196 and do not need to be routed with the same strict

considerations as the high speed TMDS signals.

In general, it is sufficient to route each auxiliary signal as a

single-ended trace. These signals are not sensitive to impedance

discontinuities, do not require a reference plane, and can be

routed on multiple layers of the PCB. However, it is best to

follow strict layout practices whenever possible to prevent the

PCB design from affecting the overall application. The specific

routing of the HPD, CEC, and DDC lines depends upon the

application in which the AD8196 is being used.

For example, the maximum speed of signals present on the

bus should suffice. The HDMI 1.3 specification, however, places

a strict 50 pF limit on the amount of capacitance that can be

measured on either SDA or SCL at the HDMI input connector.

This 50 pF limit includes the HDMI connector, the PCB, and

whatever capacitance is seen at the input of the AD8196, or an

equivalent receiver. There is a similar limit of 100 pF of input

capacitance for the CEC line.

The parasitic capacitance of traces on a PCB increases with

trace length. To help ensure that a design satisfies the HDMI

specification, the length of the CEC and DDC lines on the PCB

should be made as short as possible. Additionally, if there is a

reference plane in the layer adjacent to the auxiliary traces in

the PCB stackup, relieving or clearing out this reference plane

immediately under the auxiliary traces significantly decreases

the amount of parasitic trace capacitance. An example of the

board stackup is shown in Figure 33.

PCB DIELECTRIC

LAYER 1: MICROSTRIP

SILKSCREEN

SILKSCREEN

PCB DIELECTRIC

PCB DIELECTRIC

LAYER 2: REFERENCE PLANE

LAYER 3: REFERENCE PLANE

LAYER 4: MICROSTRIP

W

3W

3W

REFERENCE LAYER

RELIEVED UNDERNEATH

MICROSTRIP

Figure 33. Example Board Stackup

HPD is a dc signal presented by a sink to a source to indicate

that the source EDID is available for reading. The placement of

this signal is not critical, but it should be routed as directly as

possible.

When the AD8196 is powered up, one set of the auxiliary inputs

is passively routed to the outputs. In this state, the AD8196

looks like a 100 Ω resistor between the selected auxiliary inputs

and the corresponding outputs as illustrated in Figure 27. The

AD8196 does not buffer the auxiliary signals, therefore, the

input traces, output traces, and the connection through the

AD8196 all must be considered when designing a PCB to meet

HDMI/DVI specifications. The unselected auxiliary inputs of the

AD8196 are placed into a high impedance mode when the device

is powered up. To ensure that all of the auxiliary inputs of the

AD8196 are in a high impedance mode when the device is

powered off, it is necessary to power the AMUXVCC supply as

illustrated in Figure 28.

In contrast to the auxiliary signals, the AD8196 buffers the

TMDS signals, allowing a PCB designer to layout the TMDS

inputs independently of the outputs.