LMH6552 Datasheet, PDF (15/38 Pages) Texas Instruments – 1.5-GHz Fully Differential Amplifier

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8 Detailed Description
LMH6552
SNOSAX9I – APRIL 2007 – REVISED JANUARY 2015
8.1 Overview
The LMH6552 is a fully differential current feedback amplifier with integrated output common mode control,
designed to provide low distortion amplification to wide bandwidth differential signals. The common mode
feedback circuit sets the output common mode voltage independent of the input common mode, as well as
forcing the V+ and V− outputs to be equal in magnitude and opposite in phase, even when only one of the inputs
is driven as in single to differential conversion.
8.2 Functional Block Diagram
V+
-IN
±
2.5 k
High-Aol +
Differential I/O
Amplifier ±
2.5 k
+IN
+
V+
+OUT
-OUT
VEN
Buffer
±
Vcm
Error
Amplifier
+
High
Impedance
VCM
V±
8.3 Feature Description
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 RF1 and RF2. Generally, RF1 is set equal to RF2, and RG1 equal to RG2, so that the gain is set by the ratio
RF/RG. Matching of these resistors greatly affects CMRR, DC offset error, and output balance. A maximum of
0.1% tolerance resistors are recommended for optimal performance, and the amplifier is internally compensated
to operate with optimum gain flatness with RF value of 200 Ω depending on PCB layout, and load resistance.
The output common mode voltage is set by the VCM 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 VCM pin will be passed along to the outputs, reducing the performance of the amplifier.
The LMH6552 can be configured to operate on a single 10V supply connected to V+ with V- grounded or
configured for a split supply operation with V+ = +5 V and V− = −5 V. Operation on a single 10-V supply,
depending on gain, is limited by the input common mode range; therefore, AC coupling may be required.
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