january 2008 rev 1 1/8 AN2705 application note tsh173 triple video buffer with filter for sd video evaluation board user guidelines introduction this application note describes the tsh173 evaluation board, designed to help you evaluate the tsh173 triple video buffer. this document includes: a short description of the tsh173 video buffer, including the internal block diagram a description of the evaluation board and all of its components the layout of the evaluation board about the tsh173 the tsh173 is a single supply triple video buffer featuring an internal gain of 6db and an internal low pass filter of 8.2mhz cut-off frequency for each channel to fit with standard definition requirements for video line interfaces. figure 1. tsh173 evaluation board main features of the tsh173 ? 4.5v to 5.5v single supply operation ? no input capacitor required ? r-g-b, y-pb-pr, y-c-cvbs driving ? very low harmonic distortion ? 3 channels with 6db gain buffer ? each output can drive ac- or dc-coupled 150 loads ? 3 video reconstruction filters for sd ? tested on 5v power supply ? 3 internal input dc level shifter www.st.com
tsh173 description AN2705 2/8 1 tsh173 description figure 2. tsh173 internal block diagram table 1. pin description name pin number description in1, in2, in3 1, 2, 3 input pins out1, out2, out3 8, 7, 6 output pins +vcc 4 positive supply gnd 5 ground
AN2705 evaluation board description 3/8 2 evaluation board description this board is designed to be tested with a 75 generator and 75 measurement tool. input signal must be between 0v and 1.4v so that it is not clamped. otherwise, you must replace the zero ohm resistor by a 100nf capacitor in cr1, cr2, and cr3, and add a resistor bridge to polarize the input signal at the right level as shown in figure 3 . resistor bridge footprints are located on the back of the pcb. figure 3. tsh173 input the tsh173 output can be ac coupled by a 220nf capacitor to minimize the dc component on the line. to do this, you must remove the r4, r11, and r18 zero ohm resistors and solder a 220f capacitor on c6, c8, and c10 footprints as shown in figure 4 . figure 4. tsh173 ac output
evaluation board description AN2705 4/8 if you connect a system with an input impedance other than 75 to the pcb output, a resistor footprint on the board allows you to match impedances. for example: you connect a scope with a 50 output impedance. to match impedances, you must add the following resistors: r5=2.2k , r6=130 , r7=82 the output can be ac or dc connected with this configuration. figure 5. tsh173 output impedance matching power supply correct power supply bypassing is very important for optimizing performance. a 10f and a 100nf are soldered on the board. this gives good performance. however, you can improve it by adding a 100f capacitor in c3 and placing a 560h coil instead of the zero ohm resistor in lr1. figure 6. power supply
AN2705 schematic diagram and board components 5/8 3 schematic diagram and board components figure 7. board components figure 8. full layout schematics
schematic diagram and board components AN2705 6/8 table 2. list of board components part type name on board footprint description 10 f c1 1206 bypass ceramic capacitor on v cc 100 f c2 0805 bypass ceramic capacitor on v cc c3 8mm diameter bypass chemical capacitor on v cc not connected c4 0805 bypass ceramic capacitor on v cc not connected 0 ohm cr1, cr2, cr3 0805 input capacitor replaced by a 0 ohm resistor c6,c8,c10 output capacitor 220f not connected tsh173 ic1 so8 bnc 75 j1, j3, j5 bnc input signal connectors bnc 75 j2, j4, j6 bnc output signal connectors jack j7, j8 jack 2mm supply connectors sil j9 sil sil supply connector 2.54mm pitch 0 ohm lr1 1206 coil replaced by 0 ohm resistor 75 r1, r8, r15 0805 input resistor 75 r6, r13, r20 0805 output resistor r2, r3, r9, r10, r16, r17 0805 bridge resistors not connected 0 ohm r4, r11, r18 0805 strap when output is dc coupled r5, r7, r12, r14, r19, r21 0805 optional resistors to adapt output impedance
AN2705 pcb layout 7/8 4 pcb layout 5 revision history figure 9. top layer figure 10. top layout figure 11. bottom layer figure 12. bottom layout table 3. document revision history date revision changes 31-jan-2008 1 initial release.
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