cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 37

• Manufacturer Part Number: cx25840
• Description: Video Decoder And Broadcast Audio
• Manufacturer: Conexant Systems, Inc.

CX25840/1/2/3 Data Sheet
Anti-alias Filtering
3.3.5
102284B
8/3/05
Anti-Alias Filtering
Anti-aliasing filtering is integrated within the analog input channels of the device.
The anti-alias filter is designed to reject frequencies within 6 MHz of the sampling
frequency to prevent these frequencies from producing aliases in the pass band. The
cutoff frequency is set to 8 MHz to provide the flattest possible response for PAL, and
the stop band is defined as 22.0 MHz and above where the signal is attenuated at least
25 dB. Although not shown, the time-domain impulse response avoids ringing so that
sharp luma transitions do not create ringing in the filter output. Furthermore, the
delay through this filter differs by no more than 6 ns across the range from 3.0 MHz to
5.0 MHz.
The filter is automatically tuned to the correct –3 dB frequency by using the ADC
clock as a reference. Nominally, this is about 1/3 times the ADC clock frequency. To
allow sampling at half-rate, the filter bandwidth can be halved by setting
HALF_BW_CHx to 1 in the AFE Control 1 register (0x104). Alternatively, the filter
can be completely bypassed by setting BYPASS_CHx to 1 in the AFE Control 3
(0x106) register.
Table 3-10. Gain Settings (3 of 3)
Gain[5:4]
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Conexant
Gain[3:0]
10
11
12
13
14
15
2
3
4
5
6
7
8
9
Detailed Functional Description
2.375
2.625
2.875
3.125
3.375
3.625
3.875
Gain
2.25
2.75
3.25
3.75
2.5
3.5
3
3-11