LMH6517EVAL-R1

LMH6517EVAL-R1 Evaluation Board LMH6517EVAL-R1 Evaluation Board National Semiconductor Application Note 2081 Loren Siebert October 5, 2010 30130311 FIGURE 1. LMH6517EVAL-R1 Evaluation Board General Description The LMH6517EVAL-R1 evaluation board is designed to aid in the characterization of National Semiconductor’s High Speed LMH6517 Digital Controlled Variable Gain Amplifier (DVGA). Use the evaluation board as a guide for high frequency layout and as a tool to aid in device testing and characterization. Basic Operation The LMH6517 DVGA has differential input and differential output. To aid evaluation with 50Ω, single ended test equipment the LMH6517EVAL-R1 evaluation board is shipped with transformers on both the input and output signal paths. The single ended signal path uses the IN+ and OUT+ marked connectors. The IN− and OUT− signal paths are grounded and the SMA connectors are not installed. The input and output pins of the LMH6517 will self bias to approximately mid supply (2.5V). The LMH6517EVAL-R1 board has been designed with AC coupling on both input and output signal paths to protect test equipment and to ensure proper operation of the LMH6517 DVGA. Any modifications National Semiconductor® is a registered trademark of National Semiconductor Corporation. Windows® is a registered trademark of Microsoft Corporation. to the board should preserve the operating points of the DVGA and protect sensitive test equipment. Transformers T1– T4 can provide both impedance matching as well as single ended to differential conversion. The board is shipped with 2:1 turns ratio transformers that will match 50Ω equipment with the 200Ω input impedance of the LMH6517 DVGA . Do not connect the transformer secondary winding directly to ground since this will short the DVGA input voltage to ground. For differential operation the board must be modified by hand. The copper on the grounded input must be cut as illustrated in Figure 7. and additional SMA connectors need to be soldered to the board to complete the IN- and OUT- signal paths. The LMH6517EVAL-R1 Evaluation board is 0.63” thick and uses edge mounted SMA connectors. The board is built with Emerson part #142–0701–806 end launch, nickel plated SMA connectors. The LMH6517EVAL-R1 evaluation board comes built with 1:1 balun transformers and 10 Ohm series output resistors (R42, R43, R44 and R45). A 4.7pF load capacitor is placed between the resistors and the transformer (C14 and C15). The 10 Ohm resisters and 4.7pF capacitor form a snubber circuit that reduces high frequency peaking and enhances stability. The AN-2081 © 2010 National Semiconductor Corporation 301303 www.national.com AN-2081 combination of the snubber circuit and the 1:1 balun gives very good power gain and very good OIP3 performance when driving 50Ω test equipment. The output impedance of the LMH6517 amplifier is very low (