cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 34

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

Detailed Functional Description
3.3
3.3.1
3.3.2
3-8
Input Impedance
Clamping
Analog Channel
The features described in this section are automatically configured by default, but
they can be manually configured as described below.
Each of the video inputs should be AC-coupled to the source. The minimum size of
the coupling capacitor depends on the input resistance of the first circuit block of the
signal channel. This resistance is nominally 15 kΩ
capacitor values, an active impedance boosting circuit can be activated by setting
signal DROOP_COMP_CHx to logical 1 in the AFE Control 3 register (0x106). This
increases the resistance value to the 70 kΩ–200 kΩ range. It is not recommended that
impedance boosting be used without analog clamping, because the boosting circuit
may add an offset to the signal, and this offset voltage can be as high as 200 mV. The
offset can be eliminated with clamping. For most video applications, the
DROOP_COMP_CHx bit should be enabled to use the recommended 1 µF AC-
coupling value. Impedance boosting should not be used in sound IF inputs.
Since the video inputs are AC coupled, the sync and back-porch levels of the signal at
the channel input varies with the overall average picture level. This requires extra-
voltage head room, which reduces the maximum achievable signal-to-noise ratio, both
in the ADC and the analog signal processing blocks before it. Therefore, the analog
subsystem provides a clamping feature to force the sync voltage to a predefined level.
The clamping circuit is by default enabled, but can be disabled by setting the
CLAMP_EN_CHx bit in the AFE Control 2 and 3 (0x105 and 0x106) registers to 0.
When the clamp is enabled, it pulls the input voltage toward the desired level. A clamp
strobe from the digital logic is activated during the sync pulse and remains clamped
for a minimum of 1 µs per line. Since the channel gain is variable, the clamping level
seen at the ADC output depends on the gain settings. To provide a suitable clamping
level across different gain settings, the clamping level can be set with signal
CLAMP_LEVEL_CHx[2:0] according to
Table 3-9. Clamping Levels
CLAMP_LEVEL_CHx
000
001
010
011
100
101
110
111
Conexant
Clamp Voltage (relative to
mid-level)
–0.606 V
–0.460 V
–0.348 V
–0.264 V
–1.05 V
–0.80 V
–0.2 V
Table
0 V
3-9.
.
To allow smaller coupling
Clamp Code (when gain is
–160 (out of ADC range)
CX25840/1/2/3 Data Sheet
set to 0 dB)
124
218
289
343
512
384
0
102284B
8/3/05