MAX9507 datasheet(13/29 Pages) MAXIM

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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).

Video Amplifier

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.

Comparison Between DirectDrive Output

and AC-Coupled Output

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.

Video Reconstruction Filter

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.

1.8V DirectDrive Video Filter Amplifier with

Load Detection and Dual SPST Analog Switches

______________________________________________________________________________________

INPUT

OUTPUT

2ms/div

INPUT

OUTPUT

2ms/div

Figure 4. AC-Coupled Output

Figure 5. DirectDrive Output

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