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ADV3200/ADV3201

Rev. 0 | Page 30 of 3

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.

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.

0.2

0.1

–0.1

–0.2

–0.3

–0.4

4050

7080

100

TIME (µs)

Figure 100. Video Signal with a 1 nF AC Coupling Capacitor

0.2

0.1

–0.1

–0.2

–0.3

4050

7080

100

TIME (µs)

Figure 101. Video Signal with a 100 nF AC Coupling Capacitor

0.2

0.3

0.4

0.5

0.6

0.7

0.1

–0.1

–0.2

–0.3

4050

7080

100

TIME (µs)

VREF = 0V

VCLAMP

Figure 102. Input Video Signal into Sync-Tip Clamp

0.1

0.2

0.3

0.4

0.5

–0.1

–0.2

–0.3

–0.4

–0.5

4050

7080

100

TIME (µs)

VREF = 0V

VCLAMP = –0.5V

Figure 103. AC-Coupled Video Through ADV3201, Sync-Tip Clamp Enabled

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