3

521 - 17 - 00

The resulting voltage is buffered and fed back to the GB4571

input through a non-linear V to

I converter. This feedback

current transfers charge to the GB4571's input coupling

capacitor which act as a DC reservoir for corrective level

shifts.

The non - linear V to

I converter feeds back larger corrective

currents to the GB4571's input for larger differences between

V

CLAMP

and V

IN

than it does for proportionally smaller

differences between V

CLAMP

and V

IN

voltages. The non-linear

V to

I converter thus provides for substantially faster recovery

from large changes in input signal DC offset while maintaining

the overall stability of the device during near-steady-state

operation.

A 400

Ω resistor in series with the input provides some phase

advance to improve the stability of the DC restoration feedback

loop. The ratio of the capacitors C

IN

:C

HOLD

should be

maintained at approximately 20,000:1 to ensure closed loop

stability. Additional reductions in recovery time from changes

in DC offset may be obtained by reducing the values of C

IN

and C

HOLD

.

In the application shown in Figure 1, the STROBE signal is

provided by the back porch pulse of the GS4881 sync

separator. The GS4881 generates back porch pulses which

remain at horizontal rate throughout the vertical interval.

Constant rate STROBE pulses eliminate the possibility of

introducing a DC offset due to a change in duty cycle.

DETAILED DESCRIPTION

The GB4571 is intended for video applications requiring

precision DC restoration. The GB4571's signal path consists

of a simple Darlington emitter follower for maximum bandwidth

performance. With this configuration, the GB4571’s small

signal frequency response remains with

± 0.1 dB of 0 dB out

to 80 MHz.

Optimal frequency response for the GB4571 occurs with load

capacitance in the range of 18 pF. For smaller loads, an

external capacitor can be added to maintain the bandwidth of

the device. As shown in Figure 1, a small resistor, R

OUT

, should

be included in series with the GB4571 output to obtain optimal

response flatness. For a nominal load of 18 pF, R

OUT

should

be chosen to be approximately 30

Ω.

The DC restoration function is achieved through the use of a

strobed operational transconductance amplifier (OTA). The

OTA receives its input 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 GB4571, while the other input is connected to the clamp

voltage reference. When enabled by taking STROBE low, the

OTA sources or sinks current depending on whether the

output of the GB4571 is below or above V

CLAMP

respectively.

The output current from the OTA is integrated by the hold

capacitor connected to pin 1, C

HOLD

. C

HOLD

develops a

corrective voltage during the period when the clamp is enabled

and holds the corrective voltage while the clamp is disabled.

All resistors in ohms, all capacitors in microfarads unless otherwise stated.

Fig. 1 GB4571 Typical Application Circuit

680k

1.0nF

GB4571

STROBED

DC RESTORE

1

4

2

7

VIDEO

COLOR

BURST

FILTER

0.1

6

4

8

5

2

CLAMPED

VIDEO

OUTPUT

BACK

PORCH

PULSE

0.1

0.1

0.1

0.1

GS4881

SYNC

SEPARATOR

75k

22

8

6

3

5