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AN-802APPLICATION NOTE

One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 Tel: 781/329-4700 Fax: 781/461-3113 www.analog.com

INTRODUCTIONThe introduction of digital video has revolutionizedthe consumer market over the last few years. Newtechnology has enabled the design and production of low cost, high quality equipment for capture, storage,transmission, and display of video signals. The newstandards and interfaces associated with high de nition

TV signals have increased the number of signals persource and the bandwidth requirements. Compared withthe 4 MHz to 6 MHz bandwidth required for compositevideo (CVBS) signals, the bandwidth required for HDTVcan reach hundreds of MHz depending on resolution,vertical refresh rate, and scanning mode (progressive orinterlaced). Also, in order to be able to switch betweenmore and more video sources, larger switch arrays arerequired.

Switch designers have responded to this trend bychanging the architecture of the switch cell to meetthe high bandwidth and the large number of channelrequirements. This application note describes dif ferentswitch architectures and examines the performance of commonly used video switches; a brief description of the ADG794 HDTV video switch is also included.

THE CMOS SWITCHTraditional analog switches consist of two MOS transis-tors connected in parallel, as shown in Figure 1.

Figure 1. Simpli ed Schematic of a CMOS Switch

ADG794A 2.5 V Input Signal Range Switching Solution for HDTVby Ferenc Barany

When the CTRL pin is driven with a HIGH logic level, bothQ1 and Q2 will be turned ON allowing the signal appliedto either S or D pins to pass through the structure. Whena LOW logic level is applied to the CTRL pin the structurewill act like a switch in the OFF position.

The main advantages offered by the CMOS switch arethe relatively at on resistance pro le ( RON in Figure 2)

and rail-to -rail operation that allows the switch to passor block input signals whose amplitude is less than orequal to the power supply voltage as shown in Figure 3.Signal distortion is minimized by keeping the absolutevalue of switch on resistance and the atness of theresistance characteristic over the input signal range aslow as possible.

Figure 2. On Resistance Pro le of a CMOS Switch

Figure 3. Transfer Function of a CMOS Switch

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The main disadvantage of the CMOS switching cell isits reduced bandwidth due to an increased parasiticcapacitance at the source and drain (S, D) pins. The smallsignal equivalent circuit of a CMOS switch is shown inFigure 4.

Figure 4. Small Signal Equivalent Circuit

Higher bandwidths can be achieved by reducing the onresistance and the parasitic capacitance at the sourceand drain pins.

THE NMOS SWITCHThe NMOS transistors have an inherently higher band-width than the PMOS transistors, so the simplest way toincrease the bandwidth of a CMOS switch is to removethe PMOS transistor and transform the CMOS switchinto an NMOS only switch. This will lower the parasiticcapacitance and reduce the physical size of the switchcell, allowing more switches per unit area. A typicalon resistance pro le and transfer function is shown inFigure 5 and Figure 6.

Figure 5. On Resistance Pro le of a NMOS Switch

Figure 6. Transfer Function of a NMOS Switch

Figure 6 illustrates the input signal range of an NMOSswitch compared to a CMOS switch. The input rangeis reduced from VDD to VIN MAX when an NMOS onlyswitch is employed. If the voltage applied to the switchexceeds VIN MAX , the NMOS transistor will turn off andclamp the output voltage to the VOUT MAX level as shownin Figure 7.

INPUT SIGNAL > VIN MAX

OUTPUT SIGNAL CLAMPED AT VOUT MAX

Figure 7. Output Signal Clamping Effect

When a video signal is applied to the switch and its peak-to-peak amplitude exceeds VIN MAX , the clamping willalter both the luminance and chrominance components.For the luminance component, the resolution of theimage will be reduced by limiting the maximum numberof luminance levels that can be displayed. The alterat ionof the chrominance component will affect the saturationof the colors, or even wrong colors may be displayed.This effect is illustrated in Figure 8.

Figure 8. Clamping Effect on the Output Video Signal

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To avoid distortion due to clamping, the signal rangeof the switch must be large enough to pass the maxi-mum expected input signal. This last point is veryimportant because most of the video switches avail-able on the market have an input signal range of 0 V to 2 V. This is not sufficient to meet the needsof all sources given that video signals can vary inamplitude from 0.7 V p-p to 2.5 V p-p. The uniquearchitecture of the ADG794 overcomes this limitationby providing an extended input signal range of 2.5 Vwhile preserving the high bandwidth required for HDTVapplications.

ADG794HDTV VIDEO SWITCHThe ADG794 video switch from Analog Devices consistsof four SPDT (single-pole double throw) in a compact,16- lead QSOP package. The operation of the switches iscontrolled using a 2-bit parallel inter face available at theIN and EN pins. The logic level applied to the IN pin will

control which input is selected, and the EN pin enables ordisables all switches, allowing the user to connect moreADG794s in parallel for larger switching arrays.

The following are the main features of the ADG794:

Input signal range: 0 V to 2.5 V (for 5 V supply)

Bandwidth : 300 MHz (typical)

On resistance: 5 (typical)

On resistance atness : 0.68

Supply voltage: 3 V or 5 V

Quiescent current: 1 A max

Off isolation: 65 dB (typical) Channel- to-channel crosstalk: 70 dB (typical)

Fast switching times: t ON = 7 ns, t OFF = 5 ns

The full version of the ADG794 data sheet is availableonline at www.analog.com .

The high bandwidth and the extended input voltagerange of the ADG794 allows the device to be used invideo switching applications where the signal amplitudescan be as high as 2.5 V without introducing clamping

distortions.

CONCLUSIONApart from bandwidth, power consumption, and packagesize, the input signal range of a video switch should becarefully considered when designing high performancevideo switches. The ADG794 provides a cost-effectivesolution for high performance video switches due to itshigh bandwidth, low power, and extended input signalrange.

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