MAX9507 Maxim Integrated Products, MAX9507 Datasheet - Page 13

• Manufacturer Part Number: MAX9507
• Description: 1.8V DirectDrive Video Filter Amplifier
• Manufacturer: Maxim Integrated Products

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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 highpass filter
should ideally be an order of magnitude lower in fre-
quency than the frame rate. Therefore, the series capaci-
tor must be very large, typically from 220µF to 3000µF.
For space-constrained equipment, the series capacitor
is unacceptable. Changing from a single series capaci-
tor 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 MAX9507 does not have a
series capacitor, the MAX9507 will not be damaged if
the connector is shorted to a supply or to ground (see
the Short-Circuit Protection section).
If the full-scale video signal from a video DAC is
250mV, the black level of the video signal created by
the video DAC is around 75mV. The MAX9507 shifts the
black level to near ground at the output so that the
active video is above ground and the 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.
The MAX9507 has an integrated charge pump and lin-
ear regulator to create a low-noise negative supply
from the positive supply voltage. The charge pump
inverts the positive supply to create a raw negative volt-
age that is then fed into the linear regulator filtering out
the charge-pump noise.
The actual level of the video signal varies less with a
DirectDrive output than an AC-coupled output. The
average video signal level can change greatly depend-
ing upon the picture content. With an AC-coupled out-
put, the average level will change according to the time
constant formed by the series capacitor and series
resistance (usually 150Ω). For example, Figure 4 shows
an AC-coupled video signal alternating between a
completely black screen and a completely white
screen. Notice the excursion of the video signal as the
screen changes.
With the DirectDrive amplifier, the black level is held at
ground. The video signal is constrained between -0.3V
to +0.7V. Figure 5 shows the video signal from a
DirectDrive amplifier with the same input signal as the
AC-coupled system.
Load Detection and Dual SPST Analog Switches
Comparison Between DirectDrive Output
1.8V DirectDrive Video Filter Amplifier with
______________________________________________________________________________________
and AC-Coupled Output
Video Amplifier
The MAX9507 includes an internal five-pole,
Butterworth lowpass filter to condition the video signal.
The reconstruction filter smoothes the steps and
reduces the spikes created whenever the DAC output
changes value. In the frequency domain, the steps and
spikes cause images of the video signal to appear at
multiples of the sampling clock frequency. The recon-
struction filter typically has ±1dB passband flatness of
7.3MHz and 48dB attenuation at 27MHz.
Figure 4. AC-Coupled Output
Figure 5. DirectDrive Output
0V
0V
Video Reconstruction Filter
2ms/div
2ms/div
INPUT
OUTPUT
INPUT
OUTPUT
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