LMH6550_14 Datasheet, PDF (11/25 Pages) National Semiconductor (TI) – LMH6550 Differential, High Speed Op Amp

LMH6550
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SNOSAK0H – DECEMBER 2004 – REVISED MARCH 2013
APPLICATION SECTION
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.
The LMH6550 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.
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 supplies and in a fully differential configuration. See
Figure 25 and Figure 26 for recommend circuits.
R_F1
R_O
R_G1
+
a ^VI
V_CM
-
R_G2
C_L
R_L V_O
R_O
R_F2
ENABLE
Figure 25. Typical Application
The circuit shown in Figure 25 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 LMH6550 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 80 dB, 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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