ADV7181 AD [Analog Devices], ADV7181 Datasheet - Page 91

• Manufacturer Part Number: ADV7181
• Description: Multiformat SDTV Video Decoder
• Manufacturer: AD [Analog Devices]

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.
Rev. 0 | Page 91 of 96
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:
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.
{2 × π × ( R39 || R89 ) × C93 }
–100
–120
–20
–40
–60
–80
0
100k
Figure 42. Third-Order Butterworth Filter Response
300k
1M
3M
FREQUENCY (Hz)
–1
10M
= 0.62 Hz
30M
100M
ADV7181B
300M
1G