Application Section

The LMH6550 is a fully differential amplifier designed to

provide low distortion amplification to wide bandwidth differ-

ential 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

inverting mode operational amplifiers and are the primary

signal paths. The third channel is the common mode feed-

back circuit. This is the circuit that sets the output common

mode as well as driving the V

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.

The LMH6550 is a voltage feedback amplifier with gain set

by external resistors. Output common mode voltage is set by

the V

reference and should be bypassed to ground with a 0.1 µF

ceramic capacitor. Any signal coupling into the V

passed along to the output and will reduce the dynamic

range of the amplifier.

The LMH6550 is equipped with a ENABLE pin to reduce

power consumption when not in use. The ENABLE pin floats

to logic high. If this pin is not used it can be left floating. The

amplifier output stage goes into a high impedance state

when the amplifier is disabled. The feedback and gain set

resistors will then set the impedance of the circuit. For this

reason input to output isolation will be poor in the disabled

state.

FULLY DIFFERENTIAL OPERATION

The LMH6550 will perform best when used with split sup-

plies and in a fully differential configuration. See Figure 1

and Figure 3 for recommend circuits.

The circuit shown in Figure 1 is a typical fully differential

application as might be used to drive an ADC. In this circuit

closed loop gain, (A

applications in this data sheet V

voltage presented to the circuit by the signal source. For

differential signals this will be the difference of the signals on

each input (which will be double the magnitude of each

individual signal), while in single ended inputs it will just be

the driven input signal.

The resistors R

sented with a load C

converter (ADC). When fed with a differential signal, the

LMH6550 provides excellent distortion, balance and com-

mon mode rejection provided the resistors R

are well matched and strict symmetry is observed in board

layout. With a DC CMRR of over 80dB, the DC and low

frequency CMRR of most circuits will be dominated by the

external resistors and board trace resistance. At higher fre-

quencies board layout symmetry becomes a factor as well.

Precision resistors of at least 0.1% accuracy are recom-

mended and careful board layout will also be required.

With up to 15 V

mA of linear drive current the LMH6550 makes an excellent

cable driver as shown in Figure 2. The LMH6550 is also

suitable for driving differential cables from a single ended

source.

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FIGURE 1. Typical Application

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FIGURE 2. Fully Differential Cable Driver

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FIGURE 3. Single Ended in Differential Out

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