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User's Guide
SNOA524A–April 2008–Revised May 2013
AN-1828 LMH6514 Digital Controlled Variable Gain
Amplifier Evaluation Board
1 General Description
The LMH6514 evaluation board (part # LMH6514SQEVAL) is designed to aid in the characterization of
Texas Instruments high speed LMH6514 digital controlled variable gain amplifiers (DVGA).
Use the evaluation board as a guide for high frequency layout and as a tool to aid in device testing and
characterization.
2 Basic Operation
The LMH6514 DVGA has differential inputs and differential outputs. The LMH6514 will also support single-
ended to differential conversion with no transformer required on the input. To aid evaluation with 50 Ω
single-ended test equipment the LMH6514 evaluation board provides for input and output transformers.
For driving the evaluation board from a differential source, symmetrical signal paths are provided. Both
input and output paths support fully differential signal paths. For component locations refer to the
schematic in Figure 1.
The evaluation board uses end mounted SMA connectors. On the IN+ input, resistor R1 provides input
termination. The analog supply (V
CCA
) can be from 4 V to 5.25 V. The capacitor C5 is a supply bypass
capacitor and should be low ESR ceramic. Resistors R11 and R12 as well as capacitor C4 should be left
empty.
The LMH6514 evaluation board is designed for transformers wtih DC isloation between the primary and
secondary windings. If baluns (transmission line transformers with no DC blocking) are used make sure to
have DC isolation for all transformer pads.
Transformer T1 can provide both impedance matching as well as single ended to differential conversion.
The 2:1 (4:1 impedance) transformer matches 50 Ω equipment with the 200 Ω input impedance of the
LMH6514 DVGA and there is an optional capacitor at C3 if additional stability is required. Do not connect
the transformer secondary winding directly to ground. The LMH6514 has a self biased input common
mode voltage of approximately 1.3 V. The amplifier will bias up to the optimal input common mode point.
The resistors R2, R24, and R25 are normally left empty. These resistors can be used to force the
LMH6514 input common mode to a value different than it's self biased state. Most applications will not
require this function.
If using a transmission line transformer for T1, capacitor C1 is necessary to preserve the proper input
common mode voltage. For single-ended inputs to the amplifier, see Figure 7 and Figure 8.
The LMH6514 evaluation board is shipped with a transformer to facilitate testing with single-ended
equipment. To drive the LMH6514 evaluation board with a differential signal, transformer T1 must be
removed. Then, load capacitor C1 and C16 and cut the trace connecting the capacitor C2. R1 and R5
should be loaded with appropriate valued resistors (normally 50 Ω). The C3 capacitor is not needed for
this case and the transformer pads should be shorted with a low inductance wire: pad 6 to pad 1 and pad
4 to pad 3.
On the output side of the board is transformer T2. C11 isolates the output common mode voltage from the
output transformer primary windings. The output coupling capacitors C13 and C14 are necessary for
Balun transformers that provide no DC isolation between the primary and secondary windings, and are
also necessary when driving differential loads.
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SNOA524A–April 2008–Revised May 2013 AN-1828 LMH6514 Digital Controlled Variable Gain Amplifier Evaluation
Board
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