7

FN6192.1

June 28, 2006

Implementing DC-restore and amplifying composite video

using a single +5V supply amplifier, requires attention to the

performance of the amplifier over the minimum to maximum

range of output voltage swing. The differential gain - phase

plot in Figure 6 shows the amplifier accuracy operating from

signal into a 150

Ω load. Over the output DC voltage range of

0.5V to 3.25V, differential gain and phase are less than

0.05% and 0.05° respectively and defines the optimum

output voltage range of the ISL4089. Figure 6 also shows

that as the signal level increases, a corresponding decrease

in the output DC level (min/max voltage swing) can be

level to be optimally programmed within the min/max voltage

range, according to Equation 2. The values in Figure 6 take

AC performance degrades below ~0.5V, the ISL4089

maintains DC accuracy down to 10mV.

Limiting the Output Current

No output short circuit current limit exists on these parts. All

applications need to limit the output current to less than

60mA. Adequate thermal heat sinking of the parts is also

required.

Application Information

A typical single supply application circuit using the EL1883

sync separator to generate the DC-restore hold command, is

shown in Figure 13. The ISL4089 is configured for a gain of

2, and 75

Ω input and output terminations are used for cable

driving; providing an end to end gain of 1. DC-restore is

performed during sync tip using the composite sync output

of the EL1883, which clamps the -300mV input sync tip level

to 0VDC at the ISL4089 output (Figure 15 - lower trace).

Clamping sync tip to 0VDC forces the black level, color burst

and active video to the +300mV level at the 75

Ω load in the

terminal equipment, and to +600mV at the ISL4089 output

pin. The +600mV DC offset is safely within the lower linear

range of the ISL4089 output (Figure 6 - Differential Gain -

Phase) and the 2V maximum video amplitude at the output

is safely within the upper limit. In applications where the sync

tip level can’t be guaranteed, positioning the active video

within the linear range can be accomplished using the back

porch clamp output of the EL1883 and supplying +1V to the

sync tip level to pass through to the output.

RX1

475

Ω

475

Ω

4.7µF

CX1

-

+

+

-

RXT

TTL

INPUT

VIDEO

VIDEO

INPUT

0.1µF

V+

OUT

+5V

GND

GND

HOLD

75

Ω

75

Ω

ISL4089

+

1.2V

+

10mV

0V to +4.5V

A1

A2

V1

V2

FIGURE 13. BASIC +5V APPLICATION CIRCUIT

4k

40pF

S1

ISL4089