EL4093CS datasheet(9/16 Pages) ELANTEC

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EL4093C

300 MHz DC-Restored Video Amplifier

Applications Information

Product Description

The EL4093C is a high speed DC-restore system

containing a current feedback amplifier (CFA)

and a sample

hold (SH) amplifier The CFA

offers a wide 3 dB bandwidth of 300 MHz and a

slew rate of 1500 V

ms making it ideal for high

speed video applications such as SVGA

The

CFA’s excellent differential gain and phase at

358 MHz also makes it suitable for NTSC appli-

cations Drawing only 95 mA on g5V supplies

the EL4093C serves as an excellent choice for

those applications requiring both low power and

high bandwidth

The connection between the CFA and sample

hold (the Autozero interface) has been greatly

simplified The output of the sample

hold is a

high impedance current source allowing direct

connection to the CFA inverting input for au-

tozero purposes

In addition

special circuitry

within the sample

hold provides a charge cur-

rent of 85 mA in sample mode resulting in a

sample hold current ratio (ratio of charging cur-

rent to droop current) of approx 1000000

Theory of Operation

In video applications DC restoration moves the

backporch or black level to a fixed DC reference

The EL4093C uses a CFA in feedback with a

sample

hold to provide DC restoration Figure

1 shows how the two are connected to provide

this function the SH compares the output of

the CFA to a DC reference and any difference

between them causes an output current from the

SH This ‘‘autozero’’ current is fed to the CFA

inverting input the effect of which is to move the

CFA output towards the reference voltage This

autozero mechanism settles when the CFA out-

put is one VOS away from the reference (the VOS

here refers to the SH offset voltage)

44093 – 3

Figure 1

The autozero mechanism is typically active for

only a short period of each video line Figure 2

shows a NTSC video signal along with the

EL4581C back porch output The back porch sig-

nal is used to drive the HOLD input of the

EL4093C and we see that the EL4093C is in

sample mode for only 35

ms of each line It is

during this time that the autozero mechanism at-

tempts to drive the CFA output towards the ref-

erence voltage at the same time putting a correc-

tion voltage onto the hold capacitor CHOLD

During the rest of the line (60

ms) the EL4093C is

in hold mode but DC correction is maintained by

the voltage on CHOLD

44093 – 37

Figure 2

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