Datasheet

Basic Operation

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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 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 (4:1 impedance 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 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 (<2Ω @ 50MHz). Many load conditions can be

achieved by changing out the components on the evaluation board. Not all load conditions will require a

snubber circuit, but it should be included in the final circuit design unless sufficient testing has shown it to

be unnecessary.

The capacitors C5, C7, C8 and C9 isolate the output transformer from the output SMA connectors and are

not required if the transformer provides DC blocking. The spaces marked C34, C35, C36 and C37 are left

empty by the factory. These positions are included to provide the flexibility to add extra components.

Capacitors can be added to create a low pass filter. Resistors could be placed in these locations to create

different load conditions for the amplifier.

AN-2081 LMH6517EVAL-R1 Evaluation Board SNOA553B–October 2010–Revised May 2013

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