ADV7181BBCPZ datasheet(91/96 Pages) AD

91 / 96 page

ADV7181B

Rev. 0 | Page 91 of 96

DIGITAL INPUTS

The digital inputs on the ADV7181B are designed to work with

3.3 V signals, and are not tolerant of 5 V signals. Extra compo-

nents are needed if 5 V logic signals are required to be applied

to the decoder.

ANTIALIASING FILTERS

For inputs from some video sources that are not bandwidth

limited, signals outside the video band can alias back into the

video band during A/D conversion and appear as noise on the

output video. The ADV7181B oversamples the analog inputs by

a factor of 4. This 54 MHz sampling frequency reduces the

requirement for an input filter; for optimal performance it is

recommended that an antialiasing filter be employed. The

recommended low cost circuit for implementing this buffer and

filter circuit for all analog input signals is shown in Figure 43.

The buffer is a simple emitter-follower using a single npn

transistor. The antialiasing filter is implemented using passive

components. The passive filter is a third-order Butterworth

filter with a −3 dB point of 9 MHz. The frequency response of

the passive filter is shown in Figure 42. The flat pass band up to

6 MHz is essential. The attenuation of the signal at the output of

the filter due to the voltage divider of R24 and R63 is compen-

sated for in the ADV7181B part using the automatic gain control.

The ac-coupling capacitor at the input to the buffer creates a

high-pass filter with the biasing resistors for the transistor. This

filter has a cut-off of:

{2 × π × (R39||R89) × C93}–1 = 0.62 Hz

It is essential that the cutoff of this filter be less than 1 Hz to

ensure correct operation of the internal clamps within the part.

These clamps ensure that the video stays within the 5 V range of

the op amp used.

–20

–40

–60

–80

–100

–120

100k

30M

10M

300k

300M

100M

FREQUENCY (Hz)

Figure 42. Third-Order Butterworth Filter Response

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