AD8191

Rev. 0 | Page 25 of 32

APPLICATION INFORMATION

Figure 31. Layout of the TMDS Traces on the AD8191 Evaluation Board (Only Top Signal Routing Layer is Shown)

The AD8191 is an HDMI/DVI switch featuring equalized

TMDS inputs and pre-emphasized TMDS outputs. It is in-

tended for use as a 4:1 switch in systems with long cable runs

on both the input and/or the output, and is fully HDMI 1.2a

receive-compliant.

PINOUT

The AD8191 is designed for an HDMI/DVI receiver pinout

at its input and a transmitter pinout at its output. This makes

the AD8191 ideal for use in AVR-type applications where the

designer routes both the inputs and the outputs directly to

HDMI/DVI connectors. This type of layout is used on the

AD8191 evaluation board, as shown in Figure 31. When the

AD8191 is used in receiver type applications, it is necessary to

change the ordering of the output pins on the PCB to match up

with the on-board receiver.

One advantage of the AD8191 in an AVR-type application is

that all of the high speed signals can be routed on one side (the

topside) of the board, as shown in Figure 31. In addition to

12 dB of input equalization, the AD8191 provides up to 6 dB of

output pre-emphasis that boosts the output TMDS signals and

allows the AD8191 to precompensate when driving long PCB

traces or output cables. The net effect of the input equalization

and output pre-emphasis of the AD8191 is that the AD8191 can

compensate for the signal degradation of both input and output

cables; it acts to reopen a closed input data eye and transmit a

full-swing HDMI signal to an end receiver.

The AD8191 also provides a distinct advantage in receive-type

applications because it is a fully buffered HDMI/DVI switch.

Although inverting the output pin order of the AD8191 on the

PCB requires a designer to place vias in the high speed signal

path, the AD8191 fully buffers and electrically decouples the

outputs from the inputs. Consequently, the effects of the vias

placed on the output signal lines are not seen at the input of the

AD8191. The programmable output terminations also improve

signal quality at the output of the AD8191. Therefore, the PCB

designer has significantly improved flexibility in the placement

and routing of the output signal path with the AD8191 over

other solutions.

CABLE LENGTHS AND EQUALIZATION

The AD8191 offers two levels of programmable equalization

for the high speed inputs: 6 dB and 12 dB. The equalizer of

the AD8191 is optimized for video data rates of 1.65 Gbps. It

can equalize up to 20 meters of 24 AWG HDMI cable at data rates

corresponding to the video format, 1080p.