LMH730245/NOPB

CLC730245, CLC730136 (SOIC and SOT23) Single High-Speed Buffer Evaluation Boards • General Description The CLC730245 and the CLC730136 evaluation boards are designed to aid in the characterization of National Semiconductor’s High Speed SOIC and SOT23 buffers. Use these evaluation boards as a guide for high frequency layout and as a tool to aid in device testing and characterization. The evaluation boards schematic are shown in Figure 1 and Figure 2. Refer to the product data sheets for recommendations for component values. Basic Operation These boards are a very straight forward design and are intended to evaluate the parts behavior by offering 50Ω inputs and outputs traces for connection to coaxial cables with the same impedance. For other impedances the terminating resistors can be modified to help match different impedances. Both board and part are designed to handle very high frequencies. By changing the resistor connected to the input pin and ground the input impedance can be matched but keep in mind that the input trace is designed for 50Ω. By using e.g. 75Ω impedance, the results will be noticeable different especially for the highest frequencies. Also at the output it is possible to match different impedances but the output trace is designed for 50Ω. With the series resistor at the output the output cable can be matched, however this is not necessary. It is absolutely necessary to have the impedance at the end of the connected cable matched to the cable impedance but not the resistor at the beginning. Having the output series resistor matched to the cable impedance gives only an additional attenuation of −6dB. The SMA connectors are optimized for 50Ω operation. Even with optimal layout, board parasitics play a large part in high frequency performance and different termination resistors will change the frequency of the dominant parasitic poles/zeros. Layout Considerations Printed circuit board layout and supply bypassing play major roles in determining high frequency performance. When designing your own board use these evaluation boards as a guide and follow these steps to optimize high frequency performance: • Use a ground plane • Include large (10µF) capacitors on both supplies. • Near the device use 10nF ceramic capacitors from both supplies to ground. © 2003 National Semiconductor Corporation MS200707 Near the device use a 0.1µF ceramic capacitor between the supplies. Minimize all trace lengths • • Terminated always all transmission lines used. The Capacitor between both supplies is recommended for best 2nd Harmonic Distortion performance. The zenerdiode between both supplies protect the device from reverse polarity supply connections under the condition these supplies have the current limitation on. Most high-speed buffers perform best with capacitive loads if a small resistor is used in series between the amplifier and the capacitor. In that case change the output series resistor to a value of about 10Ω and evaluate what the output signal will do. Determine the right value by trial and error. While using rather high capacitive loads the matching at the output become less important because the frequency response will be dramatically lower, so a flat response will be more important. See the part’s datasheets for recommended values. Sample artwork for the CLC730245 and the CLC730136 Evaluation board is included on Figure 3. Measurement Hints Do not use normal oscilloscope probes to test these circuits. The capacitive loading will change circuit performance drastically. Instead use high frequency low ohmic passive probes. These probes have a load resistance in the range of several hundred ohms until some kΩ. For best performance use ground leads as short as possible otherwise ringing on pulse shaped signals will occur. The use of active probes gives also good results if basic rules are obtained, such as the short ground leads. Another good idea to isolate the load from the device is to change the output series resistor to a value of 450Ω. While measuring with a coaxial cable in a 50Ω system an attenuation of 10x (20dB) will occur and this acts as a resistive probe of 10:1. Even the best probes will interfere with circuit operation to some degree. Also, tools, power cables, fingers etc. near the device will change measurement results. If the circuit has a good layout and therefore exhibits a stable behavior the measurement results will only slightly changed by e.g. a fingertip on the circuit. If measurement results will change dramatically it is likely the circuit exhibits oscillations or other unwanted behavior. www.national.com CLC730245, CLC730136 (SOIC and SOT23), Single High-Speed Buffer Evaluation Boards April 2003 CLC730245, CLC730136 Board Schematics 20070707 FIGURE 1. Test Board Schematic SOIC (CLC730245) www.national.com 2 CLC730245, CLC730136 Board Schematics (Continued) 20070708 FIGURE 2. Test Board Schematic SOT23 (CLC730136) 3 www.national.com CLC730245, CLC730136 Board Layout SOIC Package (CLC730245) SOT23 Package (CLC730136) 20070702 20070701 Top Silk Top Silk 20070704 20070703 Top Copper Top Copper 20070706 20070705 Bottom Copper Bottom Copper FIGURE 3. Test Board Lay-Outs www.national.com 4 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL 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, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Americas Customer Support Center Email: new.feedback@nsc.com Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 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. National Semiconductor Asia Pacific Customer Support Center Email: ap.support@nsc.com National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: jpn.feedback@nsc.com Tel: 81-3-5639-7560 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. CLC730245, CLC730136 (SOIC and SOT23), Single High-Speed Buffer Evaluation Boards Notes IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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