AD8192 datasheet(13/28 Pages) AD

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AD8192

Rev. 0 | Page 13 of 28

to 10 mA by programming the TX_OCL bit of the transmitter

settings register. The high speed outputs must be disabled if

there are no output termination resistors present in the system.

The output equalizer (pre-emphasis) can be manually confi-

gured to provide one of four different levels of high frequency

boost. The specific boost level is selected by programming the

TX_PE bits of the transmitter settings register. No specific cable

length is suggested for a particular pre-emphasis setting because

cable performance varies widely among manufacturers.

SWITCHING MODE

The AD8192 is a 2:1 HDMI/DVI source switch. The user can

select which high speed TMDS input is routed to the output by

programming the HS_CH bit of the high speed modes register and

which low speed DDC input/output is routed to the DDC common

input/output by programming the AUX_CH bit of the auxiliary

device register.

PRE-EMPHASIS

The pre-emphasized TMDS outputs precompensate the trans-

mitted signal to account for losses in systems with long cable

runs. These long cable runs selectively attenuate the high frequency

energy of the signal, leading to degraded transition times and

eye closure. Similar to a receive equalizer, the goal of the pre-

emphasis filter is to boost the high frequency energy in the signal.

However, unlike the receive equalizer, the pre-emphasis filter is

applied before the channel, thus predistorting the transmitted

signal to account for the loss of the channel. The series connection

of the pre-emphasis filter and the channel results in a flatter

frequency response than that of the channel, thereby leading to

improved high frequency energy, improved transition times,

and improved eye opening on the far end of the channel. Using

a pre-emphasis filter for compensating channel losses allows for

longer cable runs with or without a receive equalizer on the far

end of the channel. When there is no receive equalizer on the

far end of the channel, the pre-emphasis filter should allow

longer cable runs than is acceptable with no pre-emphasis. In

the case of both a pre-emphasis filter on the near end and a

receive equalizer on the far end of the channel, the allowable

cable run should be longer than either compensation could

achieve alone. The pulse response of a pre-emphasized wave-

form is shown in Figure 27. The output voltage levels and

symbol descriptions are listed in Table 7 and Table 8, respectively.

VOCM

VOSE-BOOST

VTTO

VOSE-DC

<TBIT

VOCM

VTTO

VOSE-DC

PRE-EMPHASIS OFF

PRE-EMPHASIS ON

Figure 27. Pre-Emphasis Pulse Response

Table 7. Output Voltage Levels

DC-Coupled

PE Setting

OCL Setting

Boost (dB)

IT (mA)

VOSE-DC (mV p-p)

VOSE-BOOST (mV p-p)

VOCM (V)

VH (V)

VL (V)

250

3.175

3.3

3.050

12.5

250

312.5

3.144

3.3

2.988

250

375

3.133

3.3

2.925

250

500

3.050

3.3

2.8

500

3.050

3.3

2.8

500

625

2.988

3.3

2.675

500

750

2.925

3.3

2.550

500

1000

2.8

3.3

2.3

Table 8. Symbol Definitions

Symbol

Formula

Definition

VOSE-DC

Single-ended output voltage swing after settling

VOSE-BOOST

IT × 25 Ω1

Boosted single-ended output voltage swing

VOCM (DC-Coupled)

− T

VTTO

Common-mode voltage when the output is dc-coupled

VOCM (AC-Coupled)

− T

VTTO

Common-mode voltage when the output is ac-coupled

VOCM + VOSE-BOOST/2

High single-ended output voltage excursion

VOCM − VOSE-BOOST/2

Low single-ended output voltage excursion

1 The 25 Ω resistance in the equation is the parallel combination of the on-chip 50 Ω termination resistor and the external 50 Ω termination resistor.

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