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MAX9524 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX9524 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 22 page Standard-Definition Video Filter Amplifiers with Dual SPST Switches _______________________________________________________________________________________ 9 Detailed Description The MAX9517/MAX9524 consist of a lowpass filter and an output amplifier capable of driving a standard 150Ω video load to ground. The MAX9517 has an input buffer and the MAX9524 has an input sync-tip clamp. The MAX9517/MAX9524 both have two SPST analog switches that can be used to route audio, video, or digi- tal signals. The output amplifiers provide a fixed gain of 2V/V. The MAX9517/MAX9524 filter and amplify the video DAC output. External video signals, in which the DC bias is usually not known, can be AC-coupled to the MAX9524. Input with DC Buffer (MAX9517) The input of the MAX9517 can be directly connected to the video source if the signal is approximately between ground and 1V. This specification is commonly found at the output of most video DACs. DC-coupling requires that the input signals are ground referenced so that the sync tip of composite or luma signals is within 50mV of ground. Input with Sync-Tip Clamp (MAX9524) When the bias of the incoming video signal is either unknown or not between ground and 1V (such as an external video source), use the MAX9524 to connect the video source through a 0.1µF capacitor. The VIDIN input of the MAX9524 can only handle sig- nals with a sync pulse, such as composite video and luma. An internal sync-tip clamp sets the internal DC level of the video signal. Video Filter The filter passband (±1dB) is typically 9MHz to make the device suitable for standard-definition video signals from all sources (including broadcast video and DVD). Broadcast video signals are channel limited: NTSC sig- nals have 4.2MHz bandwidth, and PAL signals have 5MHz bandwidth. Video signals from a DVD player, however, are not channel limited; therefore, the band- width of DVD video signals can approach the Nyquist limit of 6.75MHz (recommendation ITU-R BT.601-5 specifies 13.5MHz as the sampling rate for standard- definition video). Therefore, the maximum bandwidth of the signal is 6.75MHz. To ease the filtering require- ments, most modern video systems oversample by two times, clocking the video current DAC at 27MHz. Outputs The video output amplifiers can both source and sink load current, allowing output loads to be DC- or AC- coupled. The amplifier output stage needs around 300mV of headroom from either supply rail. The parts have an internal level shift circuit that positions the sync tip at approximately 300mV at the output. If the supply voltage is greater than 3.135V (5% below a 3.3V supply), each amplifier can drive two DC-cou- pled video loads to ground. If the supply is less than 3.135V, each amplifier can drive only one DC-coupled or AC-coupled video load. Shutdown The MAX9517/MAX9524 draw less than 1µA supply current when SHDN is low. In shutdown, the amplifier output becomes high impedance. Applications Information Reducing Power Consumption in the Video DACs The MAX9517/MAX9524 have high-impedance input buffers that can work with source resistances as high as 300Ω. To reduce power dissipation in the video DACs, the DAC output resistor can be scaled up in value. The reference resistor that sets the reference current inside the video DACs must also be similarly scaled up. For instance, if the output resistor is 37.5Ω, the DAC must source 26.7mA when the output is 1V. If the output resistor is increased to 300Ω, the DAC only needs to source 3.33mA when the output is 1V. There is parasitic capacitance from the DAC output to ground. That capacitance in parallel with the DAC out- put resistor forms a pole that can potentially roll off the frequency response of the video signal. For example, 300Ω in parallel with 50pF creates a pole at 10.6MHz. To minimize this capacitance, reduce the area of the signal trace attached to the DAC output as much as possible, and place the MAX9517/MAX9524 as close as possible to the video DAC outputs. _______________________________________________________________________________________ 9 IN_ SWITCH STATE 0 OFF 1ON Table 1. Logic for Analog Switches SPST Analog Switches |
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