The MAX9503 has an integrated charge pump and linear

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 voltage that is

then fed into the linear regulator, which outputs -2V. The

linear regulator filters out the charge-pump noise.

Comparison Between DirectDrive Output

vs. AC-Coupled Output

The actual level of the video signal varies less with a

DirectDrive output than an AC-coupled output. The

video signal average can change greatly depending

upon the picture content. With an AC-coupled output,

the average will change according to the time constant

formed by the series capacitor and series resistance

(usually 150

Ω). For example, Figure 1 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 2 shows the video signal from a

DirectDrive amplifier with the same input signal as the

AC-coupled system.

Video Reconstruction Filter

Before the video signal from the DAC can be amplified,

it must be lowpass filtered to smooth the steps and to

reduce 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. The MAX9503 contains

a six-pole Butterworth lowpass filter. The passband

extends to 5.5MHz, and the minimum attenuation is

35dB at 27MHz.

Short-Circuit Protection

The MAX9503 typical application circuit includes a 75

Ω

back-termination resistor that limits short-circuit current if

an external short is applied to the video output. The

MAX9503 features internal output, short-circuit protection

to prevent device damage in prototyping and applica-

tions where the amplifier output can be directly shorted.

Shutdown

The MAX9503 features a low-power shutdown mode for

battery-powered/portable applications. Shutdown

reduces the quiescent current to less than 10nA.

Connecting SHDN to ground (SGND) disables the out-

puts and places the MAX9503 into a low-power shut-

down mode. In shutdown mode, the amplifier, charge

pump, and linear regulator are turned off and the video

output impedance is 4k

Ω.

DirectDrive Video Amplifier with

Reconstruction Filter

_______________________________________________________________________________________

9

INPUT

500mV/div

OUTPUT

500mV/div

2ms/div

Figure 1. AC-Coupled Output

INPUT

500mV/div

OUTPUT

1V/div

2ms/div

0V

0V

Figure 2. DirectDrive Output