AN-42041

www.fairchildsemi.com AN-42041 5th-Order S-Video Filter/Driver Reduces NTSC/PAL System Parts Count Introduction The FMS6400 IC is an 8-pin, SOIC, 5th-order, dual video filter designed to replace bulky discrete passive filters (Figure 1) or the handful of video integrated circuits commonly required to properly reconstruct DAC-generated video signals. The FMS6400 operates from a single +5V ± 5% supply. In addition to Y/C filtering of DAC outputs, the FMS6400 contains circuitry for sync tip clamping and restoration. Further, the filters internally sum the luma and chroma channels, providing a third filtered output: the composite video signal. The FMS6400 is designed for AC- or DC-coupled output applications, but requires capacitive coupling (0.1μF) of the input signals. All output channels are fully buffered, capable of driving 2Vpp into a 150Ω load or 1Vpp into a 75Ω load. The FMS6400 also offers gain selections of 0dB or 6dB. This provides flexibility to drive directly into an ADC, limiting the number of discrete components. The FMS6400 is fully protected from load shorting, providing the maximum junction temperature of the part is not exceeded, and may drive two separate loads each. This application note contains performance results and design information collected from an FMS6400 design utilizing a four-layer PC board. The schematic of the board used to evaluate this device is shown in Figure 9. Application Board Testing Test Equipment ■ ■ ■ ■ ■ ■ ■ One power supply: 5V ±10%, 200mA maximum One 14-inch (or larger), high-resolution CRT monitor: Sony PVM-14M2U One Y/C video signal source: JVC HR-S7100UV VCR set for “Live Feed” from a local television station One HP 3577A network analyzer One Tektronix TDS640A oscilloscope One FMS6400 demo board Assorted video cables 75Ω DAC YIN X2 75Ω 220µF YOUT 75Ω 75Ω 220µF CVOUT1 VREF X2 75Ω 220µF CVOUT2 75Ω DAC CIN X2 75Ω 220µF C OUT 75Ω May be AC or DC coupled * FMS6400 also conatins sync tip clamp and feedback amplifier for ainti-clipping Figure 1. S-Video Filter Discrete Solution © 1999 Fairchild Semiconductor Corporation AN-42041 Rev. 1.0.1 www.fairchildsemi.com AN-42041 APPLICATION NOTE Procedure Do not turn the power supply on until all connections shown in Figure 2 are completed. 1. Set the power supply to 0V. Connect the power supply to the FMS6400 Demo Board. NOTE: Use the shortest possible cables (50Ω or 75Ω) for all the following video connections. 2. Connect the video source “S-Video Out” to the “YIN” and “CIN” connectors on the demo board. 3. Connect “YOUT” and “COUT” from the test board to S-Video input of the monitor. 4. Connect “CVOUT” from the test board to line A of the monitor. 5. Adjust the input voltage to 5V. Verify that the test board is not drawing excessive current (≤75mA). 6. Apply the video test signal to the test board. A highquality image should appear on the monitor screen. NOTE: It may be necessary to connect the video test signal directly into Line C of the monitor to ensure the test signal is present and of high quality. 7. Switch the monitor input to line A (Y and COUT). 8. Note the effect of the filter action on the video signal. 9. Switch the monitor input to Line B (CVOUT). 10. Note the effect of the filter action on the video signal. Monitor Signal Source COUT DC Power Supply + YOUT – +5V YIN GND YOUT CVOUT1 CVOUT2 COUT Line A CIN FMS6400 Test Board Figure 2. Connection Diagram Results The Bode plot in Figure 3 is a non-subjective measure of the FMS6400 filtering action. A network analyzer was connected to either input channel with a 200mV RMS test signal and a plot was made of the frequency response of output vs. input. The resulting amplitude vs. frequency plot demonstrates the accuracy of the FMS6400 regarding flatness of response: 3dB (cutoff) point at 7.1MHz and linear 50dB/decade rolloff above cutoff. Time domain measurements are shown in Figures 4 and 5. Note the filtering action performed on the ‘choppy’ DAC input signal. The video signal was a standard color-bar pattern. The luma channel (Figure 4) contains the video signal amplitude, while the chroma channel (Figure 5) carries the color © 1999 Fairchild Semiconductor Corporation AN-42041 Rev. 1.0.1 information. Note that the D/A artifacts are reduced by at least a factor of 10. Though not shown, the composite output is the filtered arithmetic sum of the luma and chroma channels. Figure 6 illustrates applications where the video signal is undersampled or requires a steeper rolloff. Two filters are cascaded for a 160dB/decade rolloff. Note the use of 150Ω termination resistors on U1’s output (Figure 7). This is done to reduce the loading on U1 and preserve the DC restoration capability of U2. Do not increase the termination resistor values above 240Ω. If the resistor values are increased, decrease the series capacitors proportionately. This cascading technique can be used with additional filters to obtain even steeper rolloffs with minimal effect on the -3dB point. www.fairchildsemi.com 2 AN-42041 APPLICATION NOTE 10 0 -10 -3db -1db Amplitude (dB) -20 -30 -40 -50 -60 -70 -80 -90 10k 100k 1M 10M 100M 27MHz Frequency (Hz) Figure 3. FMS6400 Bode Plot Figure 5. Chroma Channel Filtering 10 0 -10 Amplitude (dB) -20 -30 -40 -50 -60 -70 -80 -90 10k 100k 1M 10M 100M Frequency (Hz) Figure 4. Luma Channel Filtering Figure 6. FMS6400 Bode Plot (2 Parts Cascaded) 0.1μF YIN 75Ω 1 8 100μF 150Ω 0.1μF 1 8 75Ω 220μF YOUT 150Ω U1 FMS6400 (6db option) U2 FMS6400 (6db option) 6 75Ω 220μF CVOUT2 75Ω 220μF CVOUT1 0.1μF CIN 75Ω 4 5 100μF 150Ω 0.1μF 4 5 75Ω 220μF COUT May be AC- or DC-coupled 150Ω 2 7 3 2 7 3 5V 0.1μF 1μF 0.1μF 1μF Figure 7. Cascading Two FMS6400s with 6dB Gain for Increased Filtering © 1999 Fairchild Semiconductor Corporation AN-42041 Rev. 1.0.1 www.fairchildsemi.com 3 AN-42041 APPLICATION NOTE 0.1μF YIN 75Ω 1 8 0.1μF 1 8 75Ω 220μF YOUT U1 FMS6400 (6db option) U2 FMS6400 (6db option) 6 75Ω 220μF CVOUT1 75Ω 220μF CVOUT2 0.1μF CIN 75Ω 4 5 0.1μF 4 5 75Ω 220μF COUT May be AC- or DC-coupled 2 5V 0.1μF 7 3 2 7 3 1μF 0.1μF 1μF Figure 8. Cascading Two FMS6400s, 1- 0dB and 1 – 6dB Gain for Increased Filtering C1 0.1μF YIN R1 75Ω FMS6400 1 5th-Order Filter 8 R3 75Ω C3 220μF YOUT R4 75Ω + C4 220μF CVOUT1 ? – 6 R5 75Ω C5 220μF On Channel Modulator VCR or TV C6 220μF COUT May be AC- or DC-coupled C2 0.1μF CIN R2 75Ω 4 5th-Order Filter 5 R6 75Ω 5V C7 0.1μF 2 Vcc 7 Vcco C8 1μF 3 GND Figure 9. FMS6400 Typical Application Schematic © 1999 Fairchild Semiconductor Corporation AN-42041 Rev. 1.0.1 www.fairchildsemi.com 4 AN-42041 APPLICATION NOTE Comments and Precautions Figure 9 is the FMS6400 test board schematic. A PC board similar to the test board may be used for system evaluation, providing the size of the board allows for low-noise connections. The video and power connectors can be removed and direct solder connections made to the board. Capacitors C3 through C6 were specified as tantalum because of their low-parasitic elements (ESR, ESL). High-grade, lowESR electrolytic capacitors may be substituted with no loss in performance. Use care in choosing these capacitors to ensure the ESR is both low and guaranteed. As with tantalums, observe polarity when installing in the circuit. The values of input capacitors C1 and C2 are optimized for the method in which the FMS6400 restores the sync information. Changing these values has an effect on the amount of ‘tilt’ on the luma channel only. Conclusion Video systems design engineers now have an inexpensive and space-efficient solution for filtering video signals. The existing passive discrete and active solutions are no longer viable because cost and physical space limitations top the list of priorities in new and revamped designs. The FMS6400 presents a turnkey solution to the challenge of designing a system that not only performs according to specification, but also comes in well below cost and space goals. DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1.Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. © 1999 Fairchild Semiconductor Corporation AN-42041 Rev. 1.0.1 2.A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. www.fairchildsemi.com 5