EL4093C Elantec Semiconductor, Inc. (acquired by Intersil), EL4093C Datasheet - Page 9

• Manufacturer Part Number: EL4093C
• Description: 300MHz Dc-restored Video Amplifier
• Manufacturer: Elantec Semiconductor, Inc. (acquired by Intersil)

Applications Information
Product Description
The EL4093C is a high speed DC-restore system
containing a current feedback amplifier (CFA)
and a sample
offers a wide 3 dB bandwidth of 300 MHz and a
slew rate of 1500 V
speed video applications such as SVGA The
CFA’s excellent differential gain and phase at
3 58 MHz also makes it suitable for NTSC appli-
cations Drawing only 9 5 mA on
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
high impedance current source allowing direct
connection to the CFA inverting input for au-
tozero purposes In addition special circuitry
within the sample
rent of 8 5 mA in sample mode resulting in a
sample hold current ratio (ratio of charging cur-
rent to droop current) of approx 1 000 000
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
1 shows how the two are connected to provide
this function the S H compares the output of
the CFA to a DC reference and any difference
between them causes an output current from the
S H 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 V
here refers to the S H offset voltage)
hold to provide DC restoration Figure
OS
away from the reference (the V
hold (S H) amplifier The CFA
hold provides a charge cur-
s making it ideal for high
g
5V supplies
hold is a
300 MHz DC-Restored Video Amplifier
OS
9
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 3 5
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 C
During the rest of the line (60 s) the EL4093C is
in hold mode but DC correction is maintained by
the voltage on C
HOLD
Figure 1
Figure 2
EL4093C
s of each line It is
HOLD
44093 – 37
44093 – 3