GB4571-CKA データシートの表示（PDF） - Gennum -> Semtech

部品番号

コンポーネント説明

メーカー

GB4571-CKA

Video Buffer with Precision Strobed DC Restore

Gennum -> Semtech 

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, ROUT,

should be included in series with the GB4571 output to obtain

optimal response flatness. For a nominal load of 18 pF, ROUT

should be chosen to be approximately 30 Ω.

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

VCLAMP and VIN than it does for proportionally smaller

differences between VCLAMP and VIN 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.

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 VCLAMP respectively.

The output current from the OTA is integrated by the hold

capacitor connected to pin 1, CHOLD. CHOLD develops a

corrective voltage during the period when the clamp is enabled

and holds the corrective voltage while the clamp is disabled.

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 CIN:CHOLD 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 CIN

and CHOLD.

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.

VCC

0.1

VEE

0.1

VIDEO

75k

COLOR

BURST

FILTER

22

VCLAMP

0.1

2

GS4881 8

SYNC

SEPARATOR

4

5

6

680k

0.1

8

6

GB4571

2 STROBED 7

DC RESTORE

31

45

CHOLD

1.0nF

VCC

0.1

BACK

PORCH

PULSE

ROUT

CLAMPED

VIDEO

CLOAD OUTPUT

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

Fig. 1 GB4571 Typical Application Circuit

3 of 5

521 - 17 - 02

Share Link: