AD8120

Rev. 0 | Page 9 of 16

THEORY OF OPERATION

The AD8120 is a triple, digitally controlled analog delay line,

optimized for correcting delay skew between individual channels

in common wired communication media such as unshielded

twisted pair (UTP), shielded twisted pair (STP), and coaxial

cables. In these applications, the AD8120 is used to time-align

three video signals, usually RGB or YPbPr, that arrive at a

receiver at different times due to variations in total delay per

channel. Although its primary application is analog video, the

AD8120 can be applied in other systems that require variable

analog delays up to 50 ns with 0.8 ns resolution.

The three channels consist of cascaded delay sections that are

switched in such a way as to provide a total of 50 ns total delay

difference between channels with 0.8 ns resolution. A fixed

propagation delay is common to all channels, where the associated

delay is set to 0. Therefore, the delay setting for a given channel is

a measure of the relative delay among the channels, rather than

an absolute delay.

There are three options for controlling the delay: serial periph-

voltage. Two pins select the type of control: the MODE pin selects

analog or digital control, and the SER_SEL pin selects the SPI or

Table 5. Modes of Control

PD (Pin 5)

MODE (Pin 7)

SER_SEL (Pin 6)

Control Type

0

X

X

Power-down

1

0

0

1

0

1

SPI control

1

1

X

Analog control

converted internally to digital codes with 0.8 ns steps.

Each AD8120 channel has a fixed overall gain of 2 and can

drive up to four double-terminated 75 Ω cables or PCB traces.

A power-down feature can shut down the AD8120 for power

saving when not in use.

CONTROLLING THE DELAY

The delay time of each of the three channels is controlled in one

of three ways. One control option is the application of analog

is set in discrete amounts with a nominal resolution of 0.8 ns per

quantization level (or LSB), even in the analog control mode.

A delay code is assigned to each quantization level, ranging from

0 to 63 in decimal format. The means of control (analog, SPI, or

MODE and SER_SEL inputs (see Table 5). All three channels

must use the same delay control option in a given application.

It is important to note that in skew correction applications, the

metric is the relative delay between channels, not the absolute

delay. Each channel of the AD8120 exhibits a constant delay at

its zero delay setting, referred to as its propagation delay. This

propagation delay is well matched between the channels and is

subtracted out when performing skew correction. The delay

codes, therefore, ignore the constant propagation delay and

refer only to adjustable delay beyond the propagation delay.

Delay can be calculated by multiplying the delay code by 0.8 ns.

For example, setting the red delay to 8 ns (delay code = 10), the

green delay to 16 ns (delay code = 20), and the blue delay to 28 ns

(delay code = 35) produces the following relative delays: green

delayed by 8 ns relative to red, blue delayed by 20 ns relative to

red, and blue delayed by 12 ns relative to green. If an application

requires control of absolute delay, the propagation delay must be

added to the delay corresponding to the associated delay code.

SETTING THE DELAY

In most video skew compensation applications, it is best to set

the delay of the path with the longest delay to 0, and then to add

delay to the other paths to match the longest delay. In this way,

the bandwidth of each path is maximized, and the noise of each

path is minimized. Figure 16 illustrates a case where a test step

is applied simultaneously to each cable input, and the green

cable delay is the longest.

28ns

40ns

RED CABLE OUTPUT

GREEN CABLE OUTPUT

BLUE CABLE OUTPUT

Figure 16. Cable Delay Example

In the example in Figure 16, the AD8120 green delay should be

set to 0. The AD8120 red delay is then set to the delay difference

between the green and red outputs, or 40 ns. Finally, the AD8120

blue delay is set to the delay difference between the green and blue

outputs, or 28 ns. Using the digital delay codes, green delay = 0,

red delay = 50, and blue delay = 35.