LMH6550 Datasheet, PDF (12/33 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6550
SNOSAK0I – DECEMBER 2004 – REVISED JANUARY 2015
8 Detailed Description
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8.1 Overview
The LMH6550 is a fully differential amplifier designed to provide low distortion amplification to wide bandwidth
differential signals. The LMH6550, though fully integrated for ultimate balance and distortion performance,
functionally provides three channels. Two of these channels are the V+ and V− signal path channels, which
function similarly to inverting mode operational amplifiers and are the primary signal paths. The third channel is
the common-mode feedback circuit. This is the circuit that sets the output common mode as well as driving the
V+ and V− outputs to be equal magnitude and opposite phase, even when only one of the two input channels is
driven. The common-mode feedback circuit allows single-ended to differential operation.
8.2 Functional Block Diagram
V+
-IN
±
2.5 k
High-Aol +
Differential I/O
Amplifier ±
2.5 k
+IN
+
+OUT
-OUT
EN
Buffer
V+
50 k
±
Vcm
Error
Amplifier
+
Vcm
50 k
V±
8.3 Feature Description
The LMH6550 combines a core differential I/O, high-gain block with an output common-mode sense that is
compared to a reference voltage and then fed back into the main amplifier block to control the average output to
that reference. The differential I/O block is a classic, high open-loop gain stage. The high-speed differential
outputs include an internal averaging resistor network to sense the output common-mode voltage. This voltage is
compared by a separate Vcm error amplifier to the voltage on the Vocm pin. If floated, this reference is at half
the total supply voltage across the device using two 50-kΩ resistors. This Vcm error amplifier transmits a
correction signal into the main amplifier to force the output average voltage to meet the target voltage on the
Vocm pin.
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