EL4390C 데이터 시트보기 (PDF) - Elantec -> Intersil

부품명

상세내역

제조사

EL4390C

Triple 80 MHz Video Amplifier with DC Restore

Elantec -> Intersil 

EL4390C

Triple 80 MHz Video Amplifier with DC Restore

Applications Information

Circuit Operation

Each channel of the EL4390 contains a current

feedback amplifier and a TTL CMOS compatible

clamp circuit The current that the clamp can

source or sink into the non-inverting input is ap-

proximately

I e (VCLAMP b VINa) 120

So when the non-inverting input is at the same

voltage as the clamp reference no current will

flow and hence no charge is added to the capaci-

tor When there is a difference in voltage current

will flow in an attempt to cancel the error AT

THE NON-INVERTING input The amplifier’s

offset voltage and (IBb c RF) DC errors are not

cancelled with this loop It is purely a method of

adding a controlled DC offset to the signal

As well as the offset voltage error which goes up

with gain and the IBb c RF error which drops

with gain there is also the IBa error term Since

the amplifier is capacitively coupled this small

current is slowly integrated and shows up as a

very slow ramp voltage Table below shows the

output voltage drift in 60mS for various values of

coupling capacitor all assuming the very worst

IBa current

Table 1 Charge Storage Capacitor Value vs

Droop and Charging Rates

Cap Value

(nF)

Droop in

60mS (mV)

Charge in

2mS (mV)

Charge in

4mS (mV)

10

30

400

800

22

13 6

182

364

47

64

85

170

100

30

40

80

220

1 36

18

36

In normal circuit operation the picture content

will also cause a slow change in voltage across the

capacitor so at every back porch time period

these error terms can be corrected

When a signal source is being switched eg from

two different surveillance cameras it is recom-

mended to synchronize the switching with the

vertical blanking period and to drive the HOLD

pin (pin 6) low during these lines This will en-

sure that the system has been completely re-

stored regardless of the average intensity of the

two pictures

Application Hints

Figures 1 2 shows a three channel DC-restoring

system suitable for R-G-B or Y-U-V component

video or three synchronous composite signals

Figure 1 shows the amplifiers configured for non-

inverting gain and Figure 2 shows the amplifiers

configured for inverting gains Note that since

the DC-restoring function is accomplished by

clamping the amplifier’s non-inverting input

during the back porch period any signal on the

non-inverting input will be distorted For this

reason it is recommended to use the inverting

configuration for composite video since this

avoids the color burst being altered during the

clamp time period

Since all three amplifiers are monolithic they

run at the same temperature and will have very

similar input bias currents This can be used to

advantage in situations where the droop voltage

needs to be compensated since a single trim cir-

cuit can be used for all three channels A 560KX

or similar value resistor helps to isolate each sig-

nal See Figure 2 The advantage of compensat-

ing for the droop voltage is that a smaller capaci-

tor can be used which allows a larger level resto-

ration within one line See Table 1 for values of

capacitor and charge droop rates

9

Share Link: