AD8120

Rev. 0 | Page 14 of 16

APPLICATIONS INFORMATION

Most twisted pair (TP) cables used for video transmission are

designed for data communication and typically contain four

individual TP channels. Minimization of crosstalk between pairs

is of paramount importance in data communication applications.

This is accomplished by varying the twist rates (twists per unit

length) of each pair. For a given cable length, signals traveling

on pairs with relatively high twist rates have longer distances to

traverse than signals traveling on pairs with relatively low twist

rates. The longer relative distances translate into longer relative

delays and, similarly, the shorter relative distances translate into

shorter relative delays.

The delay of any TP channel is not flat over frequency, and

an equalizer is generally used at the receiver to produce an

approximately flat delay vs. frequency characteristic as well as

an approximately flat frequency response magnitude over the

bandwidth of interest. The term “group delay” is often used in

the delay vs. frequency context. When the group delay and the

magnitude response have been corrected to the best possible

degree at the receiver, the remaining signals are close approxi-

mations to those sent at the transmit end of the cable, but with

different delays with respect to the signals sent at the transmit

end. The signals, therefore, manifest different delays relative to

each other.

The relative delay difference between any two equalized signals

at the receiver is defined as delay skew, or simply skew, and is

measured in units of time. Some bundled coaxial cables also

exhibit delay skew between channels; these skew levels are

typically much smaller than those encountered among similar

length TP channels.

The AD8120 can be used with RGB and YPbPr, as well as other

video formats. Typically, three video component signals are trans-

mitted over the TP cables, with each component carried on a pair.

For example, with RGB video signals, the red, green, and blue

signals are each transmitted over one pair. If these signals are

carried over a cable with skew larger than a quarter of a pixel

time and are displayed on a video monitor, the three colors will

not be properly aligned and the skew will be visible at the vertical

edges of objects displayed on the monitor. For fractional pixel

time skew levels, a rainbow-like effect appears at the vertical

edges of the objects; for skew levels longer than a pixel time,

vertical lines are visible on the vertical edges of objects. The

vertical lines are due to one color arriving earlier or later than

the others. The best way to observe skew is to view an object

against a black background.

The AD8120 is a triple adjustable delay line, and its primary

application is to realign the received, equalized video compo-

nents. The pixel time of UXGA video with a refresh rate of

60 Hz is approximately 6.2 ns. In this case, the 0.8 ns delay

resolution of the AD8120 represents approximately 13% of

a pixel time.

TYPICAL APPLICATION CIRCUIT FOR THE AD8123

AND AD8120

Figure 25 illustrates a complete receiver application circuit using

sync-on common mode; this circuit comprises the AD8123

triple equalizer and the AD8120. The circuit receives balanced

RGB video signals over TP cable, performs cable equalization

and skew correction, and directly drives 75 Ω coaxial cable. The

6 dB voltage gain in the AD8120 compensates for the 6 dB double

termination loss incurred driving the coaxial cable. The low-pass

filter is optimized for short distances. Refer to the AD8123 data

sheet for details regarding the sync encoding and decoding.

The filter between the AD8123 and the AD8120 is a three-pole

low-pass filter (LPF) with a cutoff frequency of approximately

148 MHz; the LPF is included to provide high frequency noise

reduction. The filter shown in the application circuit performs

well for short to medium length cables. Note that the 1 pF capaci-

tance of each AD8120 input is added to each filter capacitor that

is connected to each AD8120 input. Thus, for the filter shown,

the actual filter capacitance at each AD8120 input is 16 pF.

For longer cables, where much greater high frequency gain is

required from the AD8123, it may be desirable to scale the LPF

bandwidth back to provide greater noise reduction. This can be

done by simply scaling the inductor and capacitor values by the

ratio of the existing cutoff frequency of 140 MHz to the desired

new cutoff frequency. For example, if a new cutoff frequency of

100 MHz is desired, the inductor and capacitor values are scaled

by a factor of (140 MHz/100 MHz) = 1.4. This is summarized in

Table 13.

Table 13. Low-Pass Filter Component Selection

for 100 MHz Cutoff

Original Value

Scale

Factor

New Value

Ideal

Standard

5.6 pF

1.4

7.8 pF

7.5 pF

150 nH

1.4

210 nH

220 nH

15 pF + 1 pF1 = 16 pF

1.4

22.4 pF − 1 pF1 = 21.4 pF

22 pF