MAX9503
Detailed Description
The MAX9503 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 negative so that the
video signal black level can be placed at ground. The
MAX9503 features a six-pole, Butterworth filter to per-
form 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 MAX9503 does not have a
series capacitor, the MAX9503 will not be damaged if
the connector is shorted to a supply or to ground (see
the Short-Circuit Protection section).
Video Amplifier
Typically, the black level of the video signal created by
the video DAC is around 300mV. The MAX9503 shifts
the black level to ground at the output so that the active
video is above ground, and sync is below ground. The
amplifier needs a negative supply for its output stage to
remain in its linear region when driving sync below
ground.
DirectDrive Video Amplifier with
Reconstruction Filter
8 _______________________________________________________________________________________
Pin Description
