ADV7183ABST datasheet(101/104 Pages) AD

101 / 104 page

ADV7183A

Rev. A | Page 101 of 104

Digital Inputs

The digital inputs on the ADV7183A were designed to work

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

components 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 ADV7183A 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 41.

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 -3dB point of 9MHz. The frequency response of the

passive filter is shown in Figure 40. 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 ADV7183A 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

100kHz

30MHz

10MHz

3MHz

1MHz

300kHz

300MHz

1GHz

100MHz

FREQUENCY

Figure 40. Third-Order Butterworth Filter Response

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