ADV3201 AD [Analog Devices], ADV3201 Datasheet - Page 30

• Manufacturer Part Number: ADV3201
• Manufacturer: AD [Analog Devices]

ADV3200/ADV3201
Sync-Tip Clamp for AC-Coupled Inputs
The ADV3200/ADV3201 sync-tip clamp, when enabled, clamps
the most negative voltage of the video to equal VCLAMP. This
provides the correct dc level to the crosspoint switch and
ensures that, regardless of average picture level, the dynamic
range requirement is only the maximum input signal swing.
A basic method for ac coupling the input is to provide a series
capacitor at the input of the ADV3200/ADV3201. If a termina-
tion is provided, locate it before the series coupling capacitor.
Place the series coupling capacitor as close to the input pin as
possible.
It is important to choose the correct value for the ac coupling
capacitor at the input to the ADV3200/ADV3201. Too small a
value generates unacceptable droop as shown in Figure 100.
Using a large enough value, such as a 100 nF ac coupling
capacitor, prevents this droop, as shown in Figure 101.
–0.1
–0.2
–0.3
–0.4
–0.1
–0.2
–0.3
0.2
0.1
0.2
0.1
0
0
Figure 101. Video Signal with a 100 nF AC Coupling Capacitor
0
0
Figure 100. Video Signal with a 1 nF AC Coupling Capacitor
10
10
20
20
30
30
40
40
TIME (µs)
TIME (µs)
50
50
60
60
70
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80
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90
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100
100
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The sync-tip clamp is enabled or disabled by the sync-tip clamp
enable bit in the 193-bit word used to serially program the
ADV3200/ADV3201. The sync-tip clamp enable bit turns the
clamp function on or off for all channels; there is no clamp
on/off control for individual channels. The sync-tip clamp
function works only with signals that contain sync-tips, such as
composite video. Signals that do not have sync-tips appear
distorted if they are run through the clamp function.
The range of VCLAMP is −1 V to +0.3 V for the ADV3200
at ±2.5 V operation, and −0.5 V to +0.3 V for the ADV3201
at ±3.3 V operation. If driving VCLAMP externally, refer to
Figure 14 for the input circuitry. Note that the VCLAMP pin
has a 6 kΩ resistor tied to an on-chip VREF buffered voltage
and a 50 μA current source that sets VCLAMP nominally to
300 mV below VREF. It is recommended that bypassing be
added on the VCLAMP pin, because noise and offsets can be
injected through this pin.
Figure 103. AC-Coupled Video Through ADV3201, Sync-Tip Clamp Enabled
–0.1
–0.2
–0.3
–0.1
–0.2
–0.3
–0.4
–0.5
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.5
0.4
0.3
0.2
0.1
0
0
0
0
VREF = 0V
10
10
Figure 102. Input Video Signal into Sync-Tip Clamp
20
20
VCLAMP
VCLAMP = –0.5V
VREF = 0V
30
30
40
40
TIME (µs)
TIME (µs)
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