LMH6522

Application Note 2072 LMH6522 Evaluation Board

Literature Number: SNOA552

LMH6522 Evaluation Board National SemiconductorApplication Note 2072Loren SiebertJuly 29, 2011

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FIGURE 1. LMH6522EVAL Evaluation Board

General DescriptionThe LMH6522EVAL evaluation board is designed to aid in thecharacterization of National Semiconductor’s High SpeedLMH6522 Digital Controlled Variable Gain Amplifier (DVGA).

Use the evaluation board as a guide for high frequency layoutand as a tool to aid in device testing and characterization.

Basic OperationThe LMH6522 DVGA has differential inputs and differentialoutputs. The evaluation board has been designed to easilyinterface with 50Ω single ended test equipment. TheLMH6522EVAL evaluation board is shipped with input andoutput transformers installed to convert the DVGA differentialinputs and outputs to single ended signal paths. As built, thesignal path uses the IN+ and OUT− marked connectors. TheIN− and OUT+ signal paths are grounded. The signal pathsare fully symmetrical.

To preserve proper bias voltages there are DC blocking ca-pacitors on both the input and output signal traces. The input

pins of the LMH6522 will self bias to approximately mid supply(2.5V). The output pins need to be biased to near ground po-tential. Inductors are installed on the evaluation board toprovide proper output biasing. The bias current is approxi-mately 36 mA per output pin. Capacitors between the ampli-fier and the output transformer will prevent offset currentsfrom flowing through the transformer primary coil. Many trans-formers will show increased distortion products when there isa DC current flowing through the primary coil.

Transformers TINA– TIND can provide both impedancematching as well as single ended to differential conversion.The board is shipped with 2:1 impedance ratio transformers(1.4:1 Voltage ratio) that will match 50Ω equipment with the100Ω input impedance of the LMH6522 DVGA .

On the output side of the board are transformers TOUTA -TOUTD. The output transformers were chosen to provide agood compromise between distortion performance and phys-ical size. The LMH6522 is capable of driving a wide range ofload impedances. A 200 Ohm load impedance was chosenfor the evaluation board to emulate performance with a 100

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Ohm back terminated filter. Other configurations are possiblewith minor rework of the evaluation board.

Capacitors CCOA+/− through CCOD+/− are installed to iso-late the DVGA outputs from the output transformer primarywindings. The output resistors are 40.2 Ohm matching resis-tors. The output impedance of the LMH6522 amplifier is verylow (10Ω @ 50MHz), and the 40.2 Ohm resistors provide ter-mination for the 100 Ohm load presented by the transformer

when the evaluation board is connected to 50 Ohm test equip-ment. The JTX–2–10T output transformers have a 1:2impedance ratio.

The LMH6522 DVGA is configured to have a maximum gainof 26dB. The transformers and matching resistors contributea loss of approximately 7.5 dB. Gain through the board shouldmeasure approximately 18.6 dB.

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FIGURE 2. Output Schematic

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FIGURE 3. Input Schematic

Zoomed in portions of the input and output schematics areshown above in Figure 2 and Figure 3. The these schematicsshow that the evaluation board, as shipped, has been builtwith single ended inputs and outputs. The full signal path

schematic is shown in Figure 10 and the full evaluation boardschematic, including the digital control portions is availablein .PDF format upon request.

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Using with Differential Sources or

LoadsThe LMH6522EVAL board was designed to be easily modi-fied by the customer for fully differential operation. To modifythe board for differential operation it is necessary to removethe transformers and the resistors that ground the unusedsignal paths. Next, add coupling capacitors on the IN- andOUT+ signal lines, and finally place shorting jumpers acrossthe appropriate transformer pads. At this point theLMH6522EVAL board will be configured for a 100 Ohm dif-ferential signal path.

Other load conditions can also be easily matched by placingappropriate components on existing pads on the evaluationboard. When changing components remember to keep theinput path AC coupled so that the input common mode voltageis preserved.

Gain ControlFor ease of use, banks of DIP switches are set up to controlgain settings. For high speed interfacing to a logic analyzerthere are also 0.1” header strips. The pin functions for theseheaders are shown in the table below. For detailed instruc-tions on the pin functions see the LMH6522 productdatasheet.

The dip switches settings will impact the on-board impedancefor the J1 header pins. With the dip switches should be set tothe OPEN position the header pins are unterminated. Whenthe dip switches are closed the header pins are terminatedwith 49.9 Ohm resistors. This would be the preferred settingfor most high speed signal interfaces. Refer to the productdatasheet for the absolute maximum voltage and current rat-ings of the digital pins.

Header Jack Pin Assignments(CONA. CONB, CONC, COND)(Pin 1 is pin closest to inputconnectors)Header Pin Parallel Function

1 Address bit 0

2 Address bit 1

3 Address bit 2

4 Address bit 3

Header Pin Parallel Function

5 Address bit 4

6 Ground

7 Enable (Tri State Pin)

SPI™Compatible Gain Control

using the SPISU2 CardThe LMH6522_EVAL board can easily be controlled in theserial mode using National Semiconductor®'s SPISU2 USBto SPI controller card. This card and the required software areavailable on the National Semiconductor website. Directionsfor installing the USB control software and evaluation boarddrivers are in the user's guide available on the National Semi-conductor website.

To use the SPI card, the LMH6522 DVGA must be put intoserial mode. This is done by placing a shorting block on theMODE jumper pins which are located near the power con-nectors in the center of the board. Once this shorting block isin place the switches on the SWA switch block all need to beput in the OFF position. If the switches are in the ON positionthe digital lines will be grounded and the SPISU2 card will notbe able to communicate with the DVGA.

Once the SPUSI2 board drivers and TinyI2CSPI software areinstalled, connect the SPUSI2 board directly onto theLMH6522_EVAL double-row header (J1) by aligning pin 1 asshown in Figure 4. Plug the USB cable into the SPISU2 cardand the host PC. Start the TinyI2CSPI software and load theLMH6522 profile as shown in Figure 5. Additional commandscan be generated by changing the data in the MOSI column.Check the LMH6522 datasheet for details on the data to besent to the DVGA registers. The example SPI commands area good starting point for generating the desired commands.

SPISU2 Header Pins Serial Function

1 Chip Select

2 Ground

3 Clock

4 N/A

5 Serial Data Out (MISO)

6 N/A

7 Serial Data IN (MOSI)

8 — 14 N/A

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FIGURE 4. SPISU2 Card Connected to LMH6522EVAL Board

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FIGURE 5. TinyI2CSPI Settings

Board LayoutThe LMH6522EVAL board has been designed to provide ex-cellent signal integrity and has been thermally enhanced toprovide for excellent heat dissipation. The LMH6522EVALboard has balanced differential signal traces as well as pro-vision for using single ended test equipment.

The LMH6522 DVGA dissipates approximately 2.4W of pow-er. To keep the amplifier cool the LMH6522EVAL board uses8 layers of copper, many of which are solid gound planesconnected directly to the LMH6522 exposed thermal pad.This provides excellent heat dissipation and eliminates theneed for a heat sink. The board design files (in GERBER)format are available upon request.

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FIGURE 6. Evaluation Board Top Layer

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FIGURE 7. Layout Layer 2. Showing Metal Removed from under Input and Output Pins.

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FIGURE 8. Evaluation Board Bottom Layer

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FIGURE 9. Logic Header Schematic

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Additional Design ToolsThe RD-179: High-IF Sub-sampling Receiver Subsysemboard (SP16160CH1RB) is also available. This reference de-sign includes the ADC16DV160 ADC, the LMH6517 DVGA,

and the LMK04031B precision clock conditioner. Power reg-ulation, filters and controlled impedance board layout are allprovided in this reference design. Please visit the NationalSemiconductor website for further details.

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