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 (S/H) 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
3.58 MHz also makes it suitable for NTSC appli-
g
cations. Drawing only 9.5 mA on 5V 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 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.
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 3.5 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-
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 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-
tion voltage onto the hold capacitor C
.
HOLD
During the rest of the line (60 ms) the EL4093C is
in hold mode, but DC correction is maintained by
the voltage on C
.
HOLD
put is one V away from the reference (the V
OS
OS
here refers to the S/H offset voltage).
44093–37
Figure 2
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
