3

521 - 18 - 01

With this automatic adjustment system, the need for an external

sync separator circuit to enable the clamping action is

eliminated.

The input to the GB4570 must be AC coupled using an

appropriately sized capacitor. This capacitor acts as a DC

reservoir for corrective level shifts.

Under equilibrium

conditions, the average current supplied by the comparator

output is just sufficient to balance the current discharging the

input coupling capacitor. Under dynamic conditions, where

the system is correcting for a change in the signal level, the

charging current may be in the milliamp range. Since the

corrective current is small under equilibrium conditions, the

error voltage at the comparator input is also small making

clamping to within

± 2 mV achievable.

The circuit makes use of a ‘peak hold’ capacitor, C

HOLD

, at the

output of the comparator. This gives rise to a more constant

voltage at the comparator output. This constant voltage is

translated to a more constant corrective current by an internal

50 k

Ω resistor between the comparator output and the signal

input.

To avoid excessive phase shifts and consequent instability of

the clamp feedback loop, the peak hold capacitor should be

considerably smaller (eg. 1000 times smaller) than the input

coupling capacitor. If faster clamping is desirable, the peak

hold capacitor can be removed and a smaller input coupling

capacitor employed. In this application, some distortion of the

signal tip will occur.

DETAILED DESCRIPTION

The GB4570 is intended for video applications requiring DC

restoration. The GB4570 signal path consists of a wideband

Darlington emitter follower for maximum bandwidth

performance. With this configuration, the GB4570’s small

signal frequency response remains within

±0.1 dB of 0 dB out

to 80 MHz.

Optimal frequency response for the GB4570 occurs with load

capacitance in the range of 15 pF. For smaller loads, an

external capacitor can be added to maximize the bandwidth

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

OUT

,

should be included in series with the GB4570 output to obtain

optimal response flatness. For a nominal load of 15 pF, R

OUT

should be chosen to be approximately 40

Ω.

The DC restoration function is achieved through the use of a

simple comparator. The non-inverting input of the comparator

is connected to the clamp reference voltage, while the inverting

input is connected to the GB4570 output. For output signals

more positive than the clamp reference voltage, the comparator

output is essentially open-circuit, while signal voltages more

negative than the clamp reference voltage result in the charging

of C

HOLD

, an external non-polar capacitor connected to pin 4.

The action of the comparator is to provide a positive current

which is fed back to the input under conditions where the

device output is more negative than the clamp reference

voltage. This negative feedback raises the DC level of the

input signal to the point where all signal fluctuations occur at

voltages above the clamp reference voltage.

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

Fig. 1 Typical Application Circuit

75

VIDEO

1.1nF

8

6

4

1

2

7

3

CLAMPED

VIDEO

OUTPUT

22

0.1

0.1

GB4570

SYNC TIP

CLAMP