LMH6552 Datasheet, PDF (17/38 Pages) Texas Instruments – 1.5-GHz Fully Differential Amplifier

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9 Application and Implementation
LMH6552
SNOSAX9I – APRIL 2007 – REVISED JANUARY 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The proprietary current feedback architecture of the LMH6552 offers gain and bandwidth independence with
exceptional gain flatness and noise performance, even at high values of gain, simply with the appropriate choice
of R_F1 and R_F2. Generally R_F1 is set equal to R_F2, and R_G1 equal to R_G2, so that the gain is set by the ratio R_F/R_G.
Matching of these resistors greatly affects CMRR, DC offset error, and output balance. A minimum of 0.1%
tolerance resistors are recommended for optimal performance, and the amplifier is internally compensated to
operate with optimum gain flatness with values of R_F between 270 Ω and 390 Ω depending on package
selection, PCB layout, and load resistance.
The output common mode voltage is set by the V_CM pin with a fixed gain of 1 V/V. This pin should be driven by a
low impedance reference and should be bypassed to ground with a 0.1 µF ceramic capacitor. Any unwanted
signal coupling into the V_CM pin will be passed along to the outputs, reducing the performance of the amplifier.
This pin must not be left floating.
The LMH6552 can be operated on a supply range as either a single 5V supply or as a split +5 V and −5 V.
Operation on a single 5-V supply, depending on gain, is limited by the input common mode range; therefore, AC
coupling may be required. For example, in a DC coupled input application on a single 5-V supply, with a V_CM of
1.5 V, the input common voltage at a gain of 1 will be 0.75 V which is outside the minimum 1.2-V to 3.8-V input
common mode range of the amplifier. The minimum V_CM for this application should be greater than 2.5 V
depending on output signal swing. Alternatively, AC coupling of the inputs in this example results in equal input
and output common mode voltages, so a 1.5 V V_CM would be achievable. Split supplies will allow much less
restricted AC and DC coupled operation with optimum distortion performance.
The LMH6552 is equipped with an ENABLE pin to reduce power consumption when not in use. The ENABLE
pin, when not driven, floats high (on). When the ENABLE pin is pulled low the amplifier is disabled and the
amplifier output stage goes into a high impedance state so the feedback and gain set resistors determine the
output impedance of the circuit. For this reason input to output isolation will be poor in the disabled state and the
part is not recommended in multiplexed applications where outputs are all tied together.
9.2 Typical Applications
9.2.1 Typical Fully Differential Application
In many applications, it is required to drive a differential input ADC from a single ended source. Traditionally,
transformers have been used to provide single to differential conversion, but these are inherently bandpass by
nature and cannot be used for DC coupled applications. The LMH6552 provides excellent performance as a
single-to-differential converter down to DC. Figure 45 shows a typical application circuit where an LMH6552 is
used to produce a differential signal from a single ended source.
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