LMH6551 Datasheet, PDF (12/19 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

Application Section
The LMH6551 is a fully differential amplifier designed to
provide low distortion amplification to wide bandwidth differ-
ential signals. The LMH6551, 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 feed-
back 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.
The LMH6551 is a voltage feedback amplifier with gain set
by external resistors. Output common mode voltage is set by
the V_CM pin. This pin should be driven by a low impedance
reference and should be bypassed to ground with a 0.1 µF
ceramic capacitor. Any signal coupling into the V_CM will be
passed along to the output and will reduce the dynamic
range of the amplifier.
FULLY DIFFERENTIAL OPERATION
The LMH6551 will perform best when used with split sup-
plies and in a fully differential configuration. See Figure 1
and Figure 3 for recommend circuits.
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.
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FIGURE 2. Fully Differential Cable Driver
With up to 15 V_PP differential output voltage swing and 80
mA of linear drive current the LMH6551 makes an excellent
cable driver as shown in Figure 2. The LMH6551 is also
suitable for driving differential cables from a single ended
source.
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FIGURE 1. Typical Application
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_V) = V_OUT/ V_IN = R_F/R_G. For all the
applications in this data sheet V_INis presumed to be the
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_O help keep the amplifier stable when pre-
sented with a load C_L as is typical in an analog to digital
converter (ADC). When fed with a differential signal, the
LMH6551 provides excellent distortion, balance and com-
mon mode rejection provided the resistors R_F, R_G and R_O
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-
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FIGURE 3. Single Ended in Differential Out
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