3

520 - 52 - 2

680k

10

NON-POLAR

470p

+10V

GB4551

4

2

6

5

3

7

1

8

COMPOSITE

VIDEO INPUT

COLOR BURST

FILTER

0.1

6

4

8

5

2

CLAMPED

VIDEO OUTPUT

BACK

PORCH

PULSE

10V

0V

+10V

0.1

0.1

0.1

0.1

GS4881

SYNC. SEP.

Fig.1 Clamping Operation

DETAILED DESCRIPTION

The GB4551 is intended for video applications requiring

precision DC restoration coupled with very flat frequency

response. The signal path features a wide band operational

amplifier designed to be unity gain stable. While this amplifier

is not intended to drive 75

Ω transmission lines, it is ideal for

applications where high capacitive loads, up to several

hundred picofarads, must be driven. These applications

include input and internal system buffering and DC restoration

of video signals.

Optimal frequency response for the GB4551 occurs with load

capacitances in the range of 80 pF. For smaller loads, an

external capacitor can be added to extend the bandwidth of

should be included in series with the GB4551 output to obtain

optimal response flatness. The value of this resistor should be

chosen to match the value of the load capacitance, as shown

in Figure 5.

The DC restoration function is achieved through the use of a

strobed operational transconductance amplifier (OTA), as

shown in the Simplified Block Diagram. When enabled by

taking STROBE low, the OTA sources or sinks a current

depending on whether the output of the GB4551 is below or

above ground, respectively. The output current from the OTA

is integrated by the hold capacitor connected to pin 5, CX.

The resulting voltage is buffered and fed to the non-inverting

input of the op-amp through a 30 k

Ω resistor.Thenon-inverting

input also receives the input video signal through a large

capacitor which acts as the DC reservoir for the corrective

level shift.

A 500

Ω resistor in series with the input provides some phase

advance to improve the stability of the DC restoration feedback

loop, and provides fast recovery for offset steps as large as

±

100 mV (

± 14 IRE). The values of the input and hold capacitors

can be reduced to speed up the offset recovery time. The ratio

of the capacitors should be maintained at approximately

20,000:1 to ensure closed loop stability.

The OTA receives its inputs from a differential low pass filter.

This filter has a corner frequency of 600 kHz which attenuates

the color burst and any high frequency noise that may be

present in the signal. One input to the low pass filter senses the

output of the GB4551 while the other input is connected to the

ground reference.

In the application shown in figure 1, the input to pin 3 of the

GB4551, STROBE, is provided by the back porch output of the

GS4881 sync separator. The GS4881 generates back porch

pulses which remain at horizontal rate throughout the vertical

interval. Constant rate sample pulses eliminate the possibility

of introducing additional DC offset due to a change in duty

cycle.

The method of adjusting the DC level of the video signal used

in the GB4551 integrates over the entire back porch period.

This reduces sensitivity to noise and improves the accuracy of

the DC restoration compared with systems using sample and

hold techniques. Typical clamping accuracy of

±1 mV is

achieved by the GB4551.

TEST CIRCUITS

All resistors in ohms, all capacitors

in microfarads unless otherwise stated.