EL4089

FN7158 Rev.0.00

Page 7 of 9

July 1998

Typical Application

The EL4089 can be used to DC-restore a video waveform (see

Figure 1). The circuit forces the cable driving video amplifier’s

output to ground when the HOLD pin is at a logic low.

The “correction voltage” is stored on capacitor CX1, an

external ceramic capacitor. The capacitor value is chosen from

the system requirements. The typical input bias current to the

video amplifier is 1µA, so for a 62µs hold time, and a 0.01µF

capacitor, the output voltage drift is 6.2mV in one line.

The S/H amplifier can provide a typical current of 300µA to

charge capacitor CX1, so with a 1.2µs sampling time, the

output can be corrected by 36mV in each line.

Using a smaller value of CX1 increases both the voltage that

can be corrected, and the drift while being held, likewise, using

a larger value of CX1, reduces the voltages.

The RX1 resistor is in the circuit purely to simulate some

external source impedance, and is not needed as a real

component. Likewise for RX2. The 75

 back terminating

resistor RXT is recommended when driving 75

 cables.

The board layout should have a ground plane underneath the

EL4089, with the ground plane cut away from the vicinity of the

VIN- pin, (pin 1). This helps to minimize the stray capacitance

on pin 1.

Power supply bypassing is important, and a 0.1µF ceramic

capacitor, from each power pin to ground, placed very close to

the power pins, together with a 4.7µF tantalum bead capacitor,

is recommended.

When both digital and analog grounds are on the same board,

the EL4089 should be on the analog ground. The digital

ground can be connected to the Analog ground through a

100

-300 resistor, near the EL4089. This allows the digital

signal a return path, while preventing the digital noise from

corrupting the analog ground.

For best results the source impedance should be kept low,

using a buffer for example.

The S/H amplifier is current output and causes a small load on

the input source during sampling. When sampling is done on

the back porch with color burst there will be a small load on the

color burst. Therefore for best performance, the input should

be driven from an amplifier output or a 75

cable with75

termination.

TABLE OF CHARGE STORAGE CAPACITOR VS DROOP

CHARGING RATES (NOTE)

CAP VALUE

nF

DROOP IN

60µs

mV

CHARGE IN

1.2µs

mV

CHARGE IN

4µs

mV

10

6

36

120

33

1.8

11

36

100

0.6

3.6

12

NOTE: Basic formulae are: V (droop) = Ib+ * (Line time - Sample

time) / Capacitor and V (charge) = IOUT * Sample time / Capacitor

FIGURE 1.