LMH6551Q Datasheet, PDF (12/20 Pages) Texas Instruments – Differential, High Speed Op Amp

CMRR
Balance Error
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Application Section
The LMH6551Q is a fully differential amplifier designed to
provide low distortion amplification to wide bandwidth differ-
ential signals. The LMH6551Q, though fully integrated for
ultimate balance and distortion performance, functionally pro-
vides 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 op-
posite phase, even when only one of the two input channels
is driven. The common mode feedback circuit allows single
ended to differential operation.
The LMH6551Q 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 LMH6551Q will perform best when used with split sup-
plies and in a fully differential configuration. See Figure 1 and
Figure 3 for recommend circuits.
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The circuit shown in Figure 1 is a typical fully differential ap-
plication 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 appli-
cations in this data sheet V_IN is 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 sig-
nal.
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
LMH6551Q 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 re-
sistors and board trace resistance. At higher frequencies
board layout symmetry becomes a factor as well. Precision
resistors of at least 0.1% accuracy are recommended and
careful board layout will also be required.
FIGURE 1. Typical Application
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FIGURE 2. Fully Differential Cable Driver
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