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(R)
APPLICATION NOTE TSH10/TSH11/TSH150/TSH151 VIDEO DISTRIBUTOR
by S. LAFFONT
. . . .
FEATURES 4 outputs, 75 impedance 75 or High Z input impedance 12V power supply, 50mA 20Hz to 30MHz (-3dB) bandwidth
SCHEMATIC This application uses a high speed Operational amplifier : TSH150 or TSH151. These two Operational amplifiers have a high gain bandwidth product, typically 150MHz, and a large output current, 100mA. Due to these characteristics, it is possible to drive at a 2V peak-to-peak output a minimum of 4 video lines, each representing a 150 load. Major difference between TSH150 and TSH151 is the input impedance. The TSH150 has bipolar inputs with a medium input resistance. The TSH151 has MOSFET inputs with a very high input resistance.
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In the video distributor application both circuits can be used. The bias resistor R3 can have a very high value if a TSH151 is used, for example 100k and the input capacitor can be a small one (C1= 0.1F). As the input bias current is not negligible with the TSH150 (5Amax), it is recommended to use a medium value resistor for R3 (4.7k for example). The input capacitor C1 must have a higher value to not attenuate the low frequencies, a 2.2F is a good choice. On the schematic below, a single supply (+12V) is used. It is necessary to bias the positive input of the operational amplifier at a DC voltage in order to have a DC output at VCC/2. As the operational amplifier is used with a gain of 2, a 3V DC bias is necessary at the input. This voltage is obtained by the resistors R5 (10k) and R11 (4.7k) and a decoupling capacitor C3 (100F/6.3V). Each output is impedance matched to 75 by the resistors R6, 7, 8, 9 and the DC voltage is removed by 47F capacitors C4, 5, 6, 7.
SCHEMATIC DIAGRAM
AN872/0299
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APPLICATION NOTE
PRECAUTIONS As with every high frequencycircuit, it is mandatory to have a decoupling capacitor close to the operational amplifier. The best solution is to use an SMD 100nF cap connected directly to the pins of the IC (C10). A series resistor in the positive rail (R10) limits the sensitivity to high frequencynoise present on the power supply. An electrolytic capacitor (C2) is used to decouple the power supply at low frequency. It can be removed if the power supply is closed to the video
distributor and has a large output capacitor. In this case resistor R10 must be short-circuited, assuming the power supply is noise-free. TSH10 and TSH11 For less demanding applications, it is possible to use the low cost TSH10/11 operational amplifiers. The TSH10/11 are basically identical to the TSH150/151 with relaxed parameters, particularly the output current. The TSH10/11 are the best choice for consumer applications.
Figure 1 - PHOTO PLOT (top view - not to scale)
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APPLICATION NOTE
Figure 2 - PHOTO PLOT (top view - not to scale)
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publ ication supersedes and replaces all infor mation previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. (c) The ST logo is a trademark of STMicroelectronics (c) 1999 STMicroelectronics - Printed in Italy - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Mexico - Morocco The Netherlands - Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. (c) http://www.st.com
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