ADA4424-6

Rev. C | Page 11 of 16

APPLICATIONS INFORMATION

CVBS OUTPUT

The composite video signal (CVBS_OUT) is produced by

passively summing the Y and C channels (see Figure 1), after

amplification by their respective gain stages. Each signal

experiences a 6.2 dB loss from the passive summation and is

subsequently amplified by 6.2 dB in the fixed stage following

the summer. The net signal gain through the composite video

path is, therefore, 0 dB, and the resulting composite signal

present at the ADA4424-6 output is the sum of Y and C with

unity gain.

CORNER FREQUENCY SELECTION

The component channels of the ADA4424-6 allow for a 1 dB

filter corner frequency of either 25 MHz or 39 MHz/34 MHz.

The FC_SEL pin operates as described in Table 5.

Table 5. ED/HD Bandwidth Selection

FC_SEL (Pin 12)

HD/ED

−1 dB Corner Frequency (Typ)

Low (0)

ED

25 MHz

High (1)

HD

39 MHz (HY); 34 MHz (Pb, Pr)

INPUT DC OFFSET CANCELLATION

The luma channels (Y_IN, HY_IN) of the ADA4424-6 are capable

of detecting and cancelling dc input offsets of up to 1.1 V. Four

distinct modes of detection/cancellation are available. These are

selected via the MODE1 and MODE0 pins. The chroma (C_IN)

and color difference (HPb_IN, HPr_IN) inputs do not support

offset cancellation. It is recommended that these inputs be ac-

coupled.

Automatic Detection/Cancellation Mode

There are two modes of automatic operation. The primary mode

samples the input signal between the rising edge of the horizon-

tal sync pulse and the start of active video (back porch), averages

the value over the sampling interval, and subtracts it from the

output signal. This is the more accurate method and is able to

reduce the input-referred offsets to less than 25 mV.

An alternate method is available for copy-protected signals, where

sampling the back porch may not provide a reliable dc average.

This method detects the input negative sync tip, and clamps it

to a fixed value (−214 mV for SD, and −300 mV for ED/HD).

Sample intervals for SD and ED are shown in Figure 16, and

the HD sample intervals are shown in Figure 17.

BLACK LEVEL

SYNC TIP SAMPLE INTERVAL

BACK PORCH SAMPLE INTERVAL

Figure 16. Back Porch and Sync Tip Sample Intervals (SD/ED)

BLACK LEVEL

SYNC TIP SAMPLE INTERVAL

BACK PORCH SAMPLE INTERVAL

Figure 17. Back Porch and Sync Tip Sample Intervals (HD)