Camera system analog video filtering techniques

In most video systems, low pass filters are included on the video output lines of the video encoders. These filters reject the high frequency noise and smooth out the rise/fall edges of the video signal coming out of a video digital-to-analog converter (DAC). Traditionally discrete passive filters have been used in such configurations. However, in most of today's video subsystems, an integrated filter amplifier follows the video DAC to clean up and amplify the video signal. Maxim as well as many other analog companies offer variety of integrated video filter amplifiers that can satisfy a wide variety of application requirements.

One major application area is the video camera market. This market segment includes CCTV, Security Camera, Digital Still Camera (DSC) and Digital Video Camcorder (DVC). The purpose of this article is to provide an easy to use guide/reference for engineers currently designing products in this camera market segment. Each filter arrangement is suited for a different DAC output specification. Most common signals that these video DACs output are CVBS (Composite-Video) and Y/C. The DC level of the signal at the DAC output, the signal amplitude, as well as the AC or DC coupling of the video signal create the eight following filter arrangements. Common power supply rails for integrated video filters are 5 V or 3.3 V.

Most of the configurations in this article use 3.3 V; however for the applications with lowest power requirements, examples 6 and 7 use the MAX9509, which can be powered by a 1.8-V or 2.5-V supply. The MAX9509 uses Direct Drive technology and can deliver a 2-Vpk-pk video signal. The integrated amplifier has a fixed gain of 8V/V. For applications in which a low power DAC can only deliver a 0.25-Vpk-pk output signal, these configurations are suitable. Nevertheless, if a DAC outputs 1-Vpk-pk , designers can still take advantage of the low power consumption of the MAX9509 and reduce the output level of the DAC by scaling down the DAC's terminating resistor.

Cookbook: Eight filter arrangements
The following eight different configurations have several common assumptions: All outputs are measured at 75 ohms. Thus, when the output shows a 1-Vpk-pk value at the load, the output of the integrated filter amplifier should be 2-Vpk-pk . Additionally, a 75 percent TV NTSC color-bar signal was used as the source for all filter examples.

In Figure 1 the video DAC's output connects to a MAX9502 Filter-Amp. The filter amplifier boosts the signal and delivers a 2-Vpk-pk DC biased CVBS signal. A highly-integrated solution, the MAX9502G consumes little board area, thus also saving space in many of the portable system designs. It can operate with a single supply of 2.5V or 3.3 V and drive a video signal into a 150Ω load to ground. A power-saving Shutdown mode reduces the supply current to just 0.01µA.

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Figure 1: Please note this is NOT a good solution if AC coupling caps are required on the video output line; in that situation designers should seek a different solution.

Figure 2 shows a video DAC delivering an AC-coupled video signal to the MAX9586. This approach is most useful when the DC bias level of signal is not known. In this circuit, the input sync-tip clamp will set the DC level. This part is capable of driving two DC-coupled video loads or a single AC-coupled 150Ω load.

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Figure 2: DAC delivering an AC-coupled video signal to MAX9586

Figure 3 is very similar to example 1 with the exception that the DAC can only output a 0.5-Vpk-pk DC biased signal. The MAX9502M is the appropriate solution because of its fixed gain of 12dB. This part can drive a 2-Vpk-pk video signal into a 150Ω load to ground.

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Figure 3: Please note this is NOT a good solution if the design requires AC coupling caps on the video output line and designers should seek different solution.

Next: Luma, Chroma and CVBS

Luma, Chroma and CVBS
Figure 4 is an interesting configuration. In certain applications DACs provide both luma (Y ) and chroma (C ) but the design has only one output line. The output should be selectable between CVBS and luma. CVBS signal can be created by using a summer circuit (combiner circuit).

The tricky part is being able to provide both type of signals on the same output line and switch between them at the appropriate time. This usually is done by implementing a 2:1 Multiplexer on the output line. Fortunately, MAX9524 video filter-amplifier, used in this example, has two integrated analog single pole switches that can be setup as a 2:1 MUX. This is very useful as one integrated chip can both select the appropriate input and filter-amplify it.

DC level is unknown because of summation of Y and C; therefore the video signal should be AC coupled in front of the filter-amplifier.

The clamp circuitry after the AC coupling cap sets the bias level.

Designer should pay close attention to the combiner circuit that creates the CVBS signal. DC offset levels of luma and chroma as well as voltage compliance level of current DAC should be taken into careful consideration.

Directly connecting Y and C together, depending on the DC bias level of each signal, could lead to a CVBS signal that goes outside the voltage compliance range of DAC.

In Figure 4 , the black level of luma is at GND, and chroma has an offset of about 300 mV. When combined the maximum amplitude of CVBS signal is about 1.3 V which is within the voltage compliance range of this particular DAC.

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Figure 4: Either waveform can be viewed at the same node depending on the setting of the analog switches

Next: Multiple outputs

Multiple outputs

Figure 5 is appropriate for designs with multiple video outputs. The MAX9512 has four different output channels and it also has a Y/C-to-CVBS mixer which creates the composite video signal from luma and chroma. Each output is capable of driving two DC-coupled video loads or an AC-coupled 150Ω load.

This chip also has SmartSleep circuitry (not shown) that can detect input signals or output loads; The SmartSleep circuits reduce power consumption by turning on/off different amplifiers. This configuration most commonly can be used to provide an S-video output as well as 2 CVBS outputs.

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Figure 5: Multiple video outputs

Figure 6 keeps power consumption to a minimum by leveraging the MAX9509, which operates from a single 1.8-V supply and consumes 11.7mW average power. Another advantage of this configuration is that the black level is almost at ground without the need for a large coupling cap on the output. Because the amplifier has an internal fixed gain of 8V/V, the DAC output should have an amplitude of 0.25 Vpk-pk . This can easily be achieved by reducing the value of the terminating resistor at the output of any DAC.

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Figure 6: Please note: Video DACs with 1Vpk-pk output signal can work in this configuration by scaling down the output terminating resistor of the DAC.

For certain applications only luma (Y ) and chroma (C ) signals are available on the output of the DAC, but the system must still deliver a CVBS signal. In such situations a common solution is to use a summer circuit (combiner circuit) to create the desired output signal. This is similar to the combiner circuit in example 4 but since the amplitude of desired CVBS is only 0.25 Vpk-pk voltage compliance levels are most likely met.

Next: Direct Drive capabilities

Direct drive
Figure 7 demonstrates the appropriate filtering-amplifying solution for a very low power application. The designer can obtain the appropriate amplitude (0.25-Vpk-pk ) by scaling down the terminating resistor at the DAC output. Since the DC bias level can be unknown depending on the signals and combiner circuit, the signal should be AC-coupled into the MAX9509. A sync-tip clamp will level shift the signal appropriately at the input. Because of the filter-amplifier's Direct Drive capabilities, the black level at the amplifier's output is sitting at around ground. This eliminates the need for large coupling caps on the output. The MAX9509 can thus drive a 2-Vpk-pk video signal into a 150Ω load.

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Figure 7: Filter-amplifying solution

For applications that require two video output signals (such as S-video), a two-channel video filter-amplifier like the MAX9583 provides a compact solution- Figure 8 . The output of this device can be AC-coupled to a 150Ω load or two DC-coupled video loads. The chip has an internal fixed gain of 2V/V.

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Figure 8: A two-channel video filter-amplifier

About the author
Ben Nader is a Strategic Applications Engineer at Maxim Integrated Products. He specializes in video-design support for Maxim's multimedia business unit. Nader has a Bachelor's Degree in electrical engineering from Oregon State University. He can be reached at .

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