LMH6551Q-Q1 Datasheet, PDF (12/25 Pages) National Semiconductor (TI) – LMH6551Q Differential, High Speed Op Amp

LMH6551Q
SNOSB95E – NOVEMBER 2011 – REVISED MARCH 2013
APPLICATION SECTION
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The LMH6551Q is a fully differential amplifier designed to provide low distortion amplification to wide bandwidth
differential signals. The LMH6551Q, 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.
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 supplies and in a fully differential configuration. See
Figure 26 and Figure 27 for recommended circuits.
R_F1
R_O
R_G1
a ^VI
+
V_CM
-
R_G2
C_L
R_L V_O
R_O
R_F2
Figure 26. Typical Application
The circuit shown in Figure 26 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_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 signal.
The resistors R_O help keep the amplifier stable when presented 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
common 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 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.
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