MAX9505
DirectDrive Video Amplifier with
Reconstruction Filter and Analog Switch
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Detailed Description
The MAX9505 completely eliminates the need for
capacitors in the video output by using Maxim’s
DirectDrive technology that includes an inverting
charge pump and linear regulator. The charge pump
and linear regulator create a clean negative supply
allowing the amplifier output to swing below ground.
The amplifier output can swing both positive and nega-
tive so that the video signal black level can be placed
at ground. The MAX9505 features a six-pole,
Butterworth filter to perform reconstruction filtering on
the video input signal from the DAC.
DirectDrive
Background
Integrated video filter/amplifier circuits operating from a
single, positive supply usually create video output sig-
nals that are level-shifted above ground to keep the
signal within the linear range of the output amplifier. For
applications in which the positive DC level shift of the
video signal is not acceptable, a series capacitor can
be inserted in the output connection in an attempt to
eliminate the positive DC level shift. The series capaci-
tor cannot truly level shift a video signal because the
average level of the video varies with picture content.
The series capacitor biases the video output signal
around ground, but the actual level of the video signal
can vary significantly depending upon the RC time con-
stant and the picture content.
The series capacitor creates a highpass filter. Since
the lowest frequency in video is the frame rate, which
can be between 24Hz and 30Hz, the pole of the high-
pass filter should ideally be an order of magnitude
lower in frequency than the frame rate. Therefore, the
series capacitor must be very large, typically from
220µF to 3000µF. For space-constrained equipment,
the series capacitor is unacceptable. Changing from a
single series capacitor to a SAG network that requires
two smaller capacitors can only reduce space and
cost slightly.
The series capacitor in the usual output connection
also prevents damage to the output amplifier if the con-
nector is shorted to a supply or to ground. While the
output connection of the MAX9505 does not have a
series capacitor, the MAX9505 will not be damaged if
the connector is shorted to a supply or to ground (see
the Short-Circuit Protection section).
Pin Description
