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 approximately 75mV. The MAX9509/MAX9510
shift 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 out-
put stage to remain in its linear region when driving sync
below ground.
The MAX9509/MAX9510 have an integrated charge
pump and linear regulator to create a low-noise nega-
tive 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
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 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
and +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 (MAX9509)
The MAX9509 includes an internal five-pole, Butterworth
lowpass filter to condition the video signal. The recon-
struction 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 reconstruction fil-
ter typically has ±1dB passband flatness of 8.1MHz and
46dB attenuation at 27MHz.
Transparent Sync-Tip Clamp
The MAX9509/MAX9510 contain an integrated, trans-
parent sync-tip clamp. When using a DC-coupled
input, the sync-tip clamp does not affect the input sig-
nal, as long as it remains above ground. When using an
AC-coupled input, the transparent sync-tip clamp auto-
matically clamps the input signal to ground, preventing
it from going lower. A small current of 2µA pulls down
on the input to prevent an AC-coupled signal from drift-
ing outside the input range of the part.
Using an AC-coupled input will result in some addition-
al variation of the black level at the output. Applying a
voltage above ground to the input pin of the device
always produces the same output voltage, regardless
of whether the input is DC- or AC-coupled. However,
since the Sync-Tip Clamp Level (V
CLP
) can vary over a
small range, the video black level at the output of the
device when using an AC-coupled input can vary by an
additional amount equal to the V
CLP
multiplied by the
DC Voltage Gain (A
V
).
MAX9509/MAX9510
1.8V, Ultra-Low Power, DirectDrive
Video Filter Amplifiers
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